Can I staff the webclient from inside Microsoft Teams?

Alas, the short answer is no.

When you create a Tab within Microsoft Teams, the web page displays inside an embedded frame. When that embedded frame points to the LibraryH3lp webclient, that creates a cross-site embedded context. LibraryH3lp’s login session cookie is intentionally configured so that browsers do not pass the login session cookie along in that kind of cross-site embedded context. This is by design and done as a security precaution to prevent cross-origin information leakage and attacks like cross-site request forgery.

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