@ErikVet Adding a website to a tab and creating an app that contains a custom tab are two fairly different things. Creating a custom tab involves adding the Teams JavaScript SDK to the hosted content page and using that SDK to communicate with Teams.
Adding a website tab on the other hand just embeds the URL you provide in a web view. it cannot communicate with Teams, and there are no expectations on behavior.
Essentially you can not just add any URL to a custom tab in app. You have to customize that page to work as part of an app. Please take a look at the Add Tabs to Microsoft Teams.
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