"Use a tenant-specific endpoint or configure the application to be multi-tenant" when signing into my Azure website

If you are an Azure administrator getting this message, it may be for the the exact reason that is listed in the error message - you can not use the common API endpoint to MSFT logins to tenant-specific applications.

In my case, I was configuring an app registration with sample code - the sample code needed to be modified with a new endpoint. I.e the following line:

let kAuthority = "https://login.microsoftonline.com/common"

needed to be changed to:

let kAuthority = "https://login.microsoftonline.com/MY_TENANT_NAME"

The tenant name for your Azure organization can be obtained by typing "Tenant Status" into the Azure search bar.

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Xamarin: The above note worked for MSAL iOS - for Xamarin MSAL Android/iOS, there was no direct way to set the authority in the main call. It needs to be chained to the interactive login call.

E.g., the sample code here:

authResult = await App.PCA.AcquireTokenInteractive(App.Scopes)
                      .WithParentActivityOrWindow(App.ParentWindow)
                      .ExecuteAsync();

Needs to be changed to this:

authResult = await App.PCA.AcquireTokenInteractive(App.Scopes)
                      .WithAuthority("https://login.microsoftonline.com/YOUR_TENANT_NAME")
                      .WithParentActivityOrWindow(App.ParentWindow)
                      .ExecuteAsync();

It turns out that my account was not actually on Azure AD, so I needed to check "Accounts in any organizational directory" under "Supported account types" on portal.azure.com

Specifically: portal.azure.com > Azure Active Directory > App registrations (preview) > Your App > Authentication > Supported account types > Accounts in any organizational directory

Enable multi-tenant using below option in azure.

portal.azure.com -> Azure Active Directory -> App registrations -> Select Your App -> Authentication -> Supported account types -> Accounts in any organizational directory (Any Azure AD directory - Multitenant)