Hi,
Please be informed that customizations for HTML5/Web Client is not supported at the moment.
You can post your requirement to the uservoice and let others vote for it. We hope the feature can be released in the future.
For your reference
Remote Desktop Web Client Customizations
https://techcommunity.microsoft.com/t5/windows-virtual-desktop/remote-desktop-web-client-customizations/m-p/938488
Thanks,
Eleven
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