You should be able to use Azure / AD / OAuth2 with the Password Grant flow from PB 2017. I've done this to access MS Dynamics API in the cloud, but you should be able to just check for a valid token returned from the token request to see if the user has access or not. One of the parameters of a Password Grant request is the resource you are trying to access, so you *MAY* have to setup a "dummy" website/service in your cloud and grant access to all PowerBuilder app users as the resource that you are trying to access. You are supposed to be able to use Microsoft Graph or Azure AD Graph as the resource but I'm not very familiar with those.
So the way it would work from PB, is you would setup a service app in Azure and get a client_id and client_secret, then get the user's id/pw either from AD or a login screen. Next you would use the OAuthClient to make a token request to the oauth2 token endpoint: https://login.windows.net/{tenant}/oauth2/token . If you are returned a valid access token in the tokenresponse, then the user has access to the application.
Here's a link to help explain the password grant:
https://blogs.msdn.microsoft.com/wushuai/2016/09/25/resource-owner-password-credentials-grant-in-azure-ad-oauth/
If you need outside help, look me up on LinkedIN: https://www.linkedin.com/in/kevin-ridley-88a40913/