I managed to get the access token in postman (After registering the app on Azure on following the required steps), but the permission for the app is set to only allow logged in users to read from Sharepoint. I can download using the rest api in the browser (where i am already logged in), but how do i successfully log in using the rest api in postman or C#, since get 'Status 403 forbidden' just using the access token (which is expected) as the file is only available for some users on Sharepoint.
Setting username and password in the body of the request does to work for me.