Microsoft Graph API in a .netcore 2.1 API without

Question

I am trying to create a .net core API with Microsoft Graph API.

So I want to call the Graph API and get the group that a specific user belongs to.

Also I do not want a user to login to the API, as the API is going to be called by a third paty app which already authenticates the user.

I have seen and tried a few .net frame work application, but all needs user login.

Can anyone suggest a .netcore API samples where this is done without user login.

I am really new to GraphAPI, so any help is appreciated.