Enable customization of the Opportunity Close form
To allow users in your organization to capture more relevant details about an opportunity closing, a system administrator can enable the customization of the Opportunity Close form.
The capability to customize the Opportunity Close form is supported only in the sales app built on the Unified Interface.
To enable customization:
In your app, on the navigation bar, select the Settings icon , and then select Advanced Settings.
The Business Management settings page opens in a new browser tab.
On the navigation bar, select Settings, and then select Administration.
Select System Settings.
In the System Settings dialog box, on the Sales tab, set the Customize close opportunity form field to Yes.