Make sure to login in your multi-tenant API with user of another Tenant using
https://login.microsoftonline.com/common/oauth2/v2.0/authorize?client_id={client-id of Tenant A API}&response_type=code&redirect_uri={redirect URI registered in Tenant A API}&response_mode=fragment&scope=openid%20offline_access%20https%3A%2F%2Fgraph.microsoft.com%2F.default&state=12345 which will ask for user's consent
Once the consent is provided, the service principal object of API of Tenant A gets registered in Tenant B. This API now can be found under Enterprise Registration Blade of AAD in Tenant B.
Here Tenant A is where you register your application.
Reference -https://learn.microsoft.com/en-us/entra/identity-platform/howto-convert-app-to-be-multi-tenant
Hope this will help.
Thanks,
Shweta
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