Accessing Microsoft Graph API without using login page

Yes this is possible. Essentially you grant access application access to Graph API instead of a user.

The documentation for such access is here:

https://developer.microsoft.com/en-us/graph/docs/concepts/auth_v2_service

You'll still need to a request a bearer token to send with all your REST requests, but the bearer token will be for the application itself and not a user.

I set this up for one of my applications using the Graph SDK for .NET, so if you need specific examples for Graph SDK for .NET let me know.

Yes, it is possible if you have the right information - all you need to do is to get a delegated access token.

Explanation:

When dealing with access to resources, Microsoft Graph has two levels of access token requirements:

• Most methods support Application only tokens, meaning once an OAuth app has consent it can access the resource whenever it wants.
• But for some methods, it is not enough (they are too sensitive for an automated process) and require a Delegated token, meaning token which contains both a valid Client and User. You can see in each method documentation which token it requires.

Normally delegated access tokens are the result of the two major OAuth flows which require user interaction (Authorization Code Grant and Implicit Grant) but you can also get them from two other flows: Resource Owner Credentials Grant and On-Behalf-Of Grant, which are both supported by Microsoft.

For a full guide on how to setup everything you need in order to use those flows (including Postman examples) you can look at my article:

Getting Access Token for Microsoft Graph Using OAuth REST API