I'm trying to use the Microsoft Graph API through the OAUTH2 Authentication however I'm struggling to work out how to use Delegated Permissions and not require a user to login.
I'm happy to authenticate the app with myself once, but this will be running on a daemon/service and won't be interacted with via a user. Because of this I can't use the way Microsoft describes Delegated Permissions as that uses /authorize first and then a call can be made to /token.
I know you can use secret keys for /token but it seems that only is using the Application Permissions and not Delegated - which is what I have access to.
Is there a way to authenticate using Delegated Permissions as if I was a user but without a user having to use a sign in page every time?
This isn't possible. The term "delegated" is very intentional here in that it means "the user has delegated their permissions to your application so you can operate on behalf of that user". Application permissions are not delegated because there is no user in context to delegate their access rights to you.
Much of the Microsoft Graph functionality works with both Application and Delegated scopes so in many cases you can still execute the same scenarios. There are some caveats such as using the shorthand /me which doesn't exist when there isn't a user authenticated (instead you need to use /users[{id}]). There are however some cases where there isn't an equivalent Application scope and these are regularly looked at in an effort to close the gap.