Ok. I've got something working. rather than using .net MAUI Web authenticator use a regular webview and follow this example https://learn.microsoft.com/en-us/azure/app-service/overview-authentication-authorization.

Then once you select your google account it will redirect you to this url that contains the jwt "{function_app}/.auth/login/done#token={your token}". and use this token to make requests to the function app by sending it in the headers 'X-ZUMO-AUTH'