Follow https://learn.microsoft.com/en-us/microsoft-365/whiteboard/configure-privacy-settings?view=o365-worldwide to enable optional connected experiences:
To configure the availability of optional connected experiences, use the Office cloud policy service in the Microsoft 365 Apps admin center. Sign in with your administrator account and go to Customization > Policy Management. The policy you want to configure is named: Allow the use of additional optional connected experiences in Office.
Note that as per https://support.microsoft.com/en-us/office/whiteboard-optional-connected-experiences-5a3b6440-2791-4258-b6a5-a0d47529386d optional connected experiences are currently only available for commercial Microsoft 365 accounts.
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hth
Marcin