How to add multi domain user emails to Azure AD

We have three different websites and we want to use Azure AD for the purpose of single sign on. My question is how do I add users to Azure AD (via API) who could have differnt emails such as foo@gmail.com, bar@yahoo.com, baz@outlook.com, etc

When I try to add users with these emails to Azure via API, I get the error:

Property userPrincipalName is invalid.

If however I add users with azure tenant name (like reinhold@mytenant.onmicrosoft.com), they are added fine.

I searched in google but to no avail.

So is there any way to add users having gmail/yahoo/outlook/other email addresses to Azure AD using API ?

Thanks