Accessing API with username and password

Hi folks, we are actually developing a Smartphone App with ionic Framework for controlling many instances of Xibo CMS as a Messaging and alerting Box for the THW (German governmental disaster relief organization). We are pretty far with our feature set actually. We managed different tasks while using the REST API with our mobile app but actually we are loggin in to the system with client_key and client_secret to obtain the auth token, everything works fine. What we actually need is a login to the API with username and password as we declared them in the Xibo Backend and than we need to obtain the auth key for accessing the API with that credentials. Is this possible? If so, how do we accomplish that task?
Thanks in advance.

This is called a password_grant in the oAuth2 specification and isn’t yet possible with Xibo. You can use the access_code grant (authorization), which allows for individual user authentication (this is the correct grant to use in most cases where you want a user to login).