Given below is the manifest.xml I want to load the webpage inside the taskpane instead of a separate web browser.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<OfficeApp xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="MailApp">
<Id>55541d9d-dc69-4aa3-8015-35f93ad19b83</Id>
<Version>1.0.0.0</Version>
<ProviderName>ExampleWebsitename</ProviderName>
<DefaultLocale>en-US</DefaultLocale>
<DisplayName DefaultValue="ExampleWebsitename"/>
<Description DefaultValue="New Creates tasks from emails"/>
<IconUrl DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-32.png"/>
<HighResolutionIconUrl DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-80.png"/>
<SupportUrl DefaultValue="https://www.ExampleWebsitename.com"/>
<AppDomains>
<!-- <AppDomain>https://release.ExampleWebsitename.net</AppDomain> -->
<AppDomain>https://ExampleWebsitename.net</AppDomain>
</AppDomains>
<Hosts>
<Host Name="Mailbox"/>
</Hosts>
<Requirements>
<Sets>
<Set Name="Mailbox" MinVersion="1.1"/>
</Sets>
</Requirements>
<FormSettings>
<Form xsi:type="ItemRead">
<DesktopSettings>
<SourceLocation DefaultValue="https://release.ExampleWebsitename.net/?mobileapp=true/#/outlook"/>
<RequestedHeight>250</RequestedHeight>
</DesktopSettings>
</Form>
</FormSettings>
<Permissions>ReadWriteItem</Permissions>
<Rule xsi:type="RuleCollection" Mode="Or">
<Rule xsi:type="ItemIs" ItemType="Message" FormType="Read"/>
</Rule>
<DisableEntityHighlighting>false</DisableEntityHighlighting>
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
<Requirements>
<bt:Sets DefaultMinVersion="1.3">
<bt:Set Name="Mailbox"/>
</bt:Sets>
</Requirements>
<Hosts>
<Host xsi:type="MailHost">
<DesktopFormFactor>
<FunctionFile resid="Commands.Url"/>
<ExtensionPoint xsi:type="MessageReadCommandSurface">
<OfficeTab id="TabDefault">
<Group id="msgReadGroup">
<Label resid="GroupLabel"/>
<Control xsi:type="Button" id="msgReadOpenPaneButton">
<Label resid="TaskpaneButton.Label"/>
<Supertip>
<Title resid="TaskpaneButton.Label"/>
<Description resid="TaskpaneButton.Tooltip"/>
</Supertip>
<Icon>
<bt:Image size="16" resid="ExampleWebsitename-logo.16x16"/>
<bt:Image size="32" resid="ExampleWebsitename-logo.32x32"/>
<bt:Image size="80" resid="ExampleWebsitename-logo.80x80"/>
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="Taskpane.Url"/>
</Action>
</Control>
</Group>
</OfficeTab>
</ExtensionPoint>
</DesktopFormFactor>
</Host>
</Hosts>
<Resources>
<bt:Images>
<bt:Image id="ExampleWebsitename-logo.16x16" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-16.png"/>
<bt:Image id="ExampleWebsitename-logo.32x32" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-32.png"/>
<bt:Image id="ExampleWebsitename-logo.80x80" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-80.png"/>
</bt:Images>
<bt:Urls>
<bt:Url id="Taskpane.Url" DefaultValue="https://release.ExampleWebsitename.net/?mobileapp=true/#/outlook"/>
</bt:Urls>
<bt:ShortStrings>
<bt:String id="GroupLabel" DefaultValue="ExampleWebsitename Add-in"/>
<bt:String id="TaskpaneButton.Label" DefaultValue="Mobile View Create Task"/>
</bt:ShortStrings>
<bt:LongStrings>
<bt:String id="TaskpaneButton.Tooltip" DefaultValue="Opens a pane displaying all available properties."/>
</bt:LongStrings>
</Resources>
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
<Requirements>
<bt:Sets DefaultMinVersion="1.3">
<bt:Set Name="Mailbox" />
</bt:Sets>
</Requirements>
<Hosts>
<Host xsi:type="MailHost">
<DesktopFormFactor>
<FunctionFile resid="Commands.Url"/>
<ExtensionPoint xsi:type="MessageReadCommandSurface">
<OfficeTab id="TabDefault">
<Group id="msgReadGroup">
<Label resid="GroupLabel"/>
<Control xsi:type="Button" id="msgReadOpenPaneButton">
<Label resid="TaskpaneButton.Label"/>
<Supertip>
<Title resid="TaskpaneButton.Label"/>
<Description resid="TaskpaneButton.Tooltip"/>
</Supertip>
<Icon>
<bt:Image size="16" resid="ExampleWebsitename-logo.16x16"/>
<bt:Image size="32" resid="ExampleWebsitename-logo.32x32"/>
<bt:Image size="80" resid="ExampleWebsitename-logo.80x80"/>
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="Taskpane.Url"/>
</Action>
</Control>
</Group>
</OfficeTab>
</ExtensionPoint>
</DesktopFormFactor>
<MobileFormFactor>
<FunctionFile resid="Commands.Url" />
<ExtensionPoint xsi:type="MobileMessageReadCommandSurface">
<Group id="mobileMsgRead">
<Label resid="GroupLabel" />
<Control xsi:type="MobileButton" id="mblTaskPaneBtn">
<Label resid="TaskpaneButton.Label" />
<Icon xsi:type="bt:MobileIconList">
<bt:Image size="25" scale="1" resid="ExampleWebsitename-logo.25x25" />
<bt:Image size="25" scale="2" resid="ExampleWebsitename-logo.25x25" />
<bt:Image size="25" scale="3" resid="ExampleWebsitename-logo.25x25" />
<bt:Image size="32" scale="1" resid="ExampleWebsitename-logo.32x32" />
<bt:Image size="32" scale="2" resid="ExampleWebsitename-logo.32x32" />
<bt:Image size="32" scale="3" resid="ExampleWebsitename-logo.32x32" />
<bt:Image size="48" scale="1" resid="ExampleWebsitename-logo.48x48" />
<bt:Image size="48" scale="2" resid="ExampleWebsitename-logo.48x48" />
<bt:Image size="48" scale="3" resid="ExampleWebsitename-logo.48x48" />
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="Taskpane.Url" />
</Action>
</Control>
</Group>
</ExtensionPoint>
</MobileFormFactor>
</Host>
</Hosts>
<Resources>
<bt:Images>
<bt:Image id="ExampleWebsitename-logo.16x16" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-16.png"/>
<bt:Image id="ExampleWebsitename-logo.25x25" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-25.png"/>
<bt:Image id="ExampleWebsitename-logo.32x32" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-32.png"/>
<bt:Image id="ExampleWebsitename-logo.48x48" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-48.png"/>
<bt:Image id="ExampleWebsitename-logo.80x80" DefaultValue="https://localhost:3000/assets/ExampleWebsitename-logo-80.png"/>
</bt:Images>
<bt:Urls>
<bt:Url id="Taskpane.Url" DefaultValue="https://release.ExampleWebsitename.net/?mobileapp=true/#/outlook"/>
</bt:Urls>
<bt:ShortStrings>
<bt:String id="GroupLabel" DefaultValue="ExampleWebsitename Outlook Add-in"/>
<bt:String id="TaskpaneButton.Label" DefaultValue="New Create Task"/>
</bt:ShortStrings>
<bt:LongStrings>
<bt:String id="TaskpaneButton.Tooltip" DefaultValue="Opens a pane displaying all available properties."/>
</bt:LongStrings>
</Resources>
</VersionOverrides>
</VersionOverrides>
</OfficeApp>