Viva Engage cannot authenticate on Teams Desktop App in my custom App

DanielWigger - This is not supported.

• First of all, you should be using the Viva Engage app to embed Engage content in Teams rather than roundtripping through SharePoint:
  https://teams.microsoft.com/l/app/db5e5970-212f-477f-a3fc-2227dc7782bf?source=app-details-dialog
• Second of all, the only officially supported mechanisms for integrating SharePoint content in Teams are the built-in SharePoint, SharePoint Pages, and Lists apps as documented here:
  https://support.microsoft.com/en-us/office/add-a-sharepoint-page-list-or-document-library-as-a-tab-in-teams-131edef1-455f-4c67-a8ce-efa2ebf25f0b 
  Or Viva Connections for a portal experience:
  https://learn.microsoft.com/en-us/viva/connections/edit-viva-home#choose-the-default-landing-experience-for-viva-connections-desktop 

Using custom apps is only supported for SPFx-based solutions and in that case, very specific guidelines need to be followed when creating the app manifest:
https://learn.microsoft.com/en-us/sharepoint/dev/spfx/deployment-spfx-teams-solutions#developer-provided-microsoft-teams-app-manifest--package 

The most important requirements are:

1. The TeamsLogon.aspx redirect should be used to handle authentication. For the sample app provided in this IcM ticket, the contentUrl should look like this:

   https://{teamSiteDomain}/_layouts/15/TeamsLogon.aspx?SPFX=true&dest=/sites/CustomerSite.aspx
   

2. A webApplicationInfo section should be added to the manifest that looks like so:

   "webApplicationInfo": {
     "resource": "https://{teamSiteDomain}",
     "id": "00000003-0000-0ff1-ce00-000000000000"
   }