Hello Oauthuser,
Thanks for reaching out. Using POST to get the properties of a group is not supported. You can find more information on the topic in article https://learn.microsoft.com/en-us/graph/api/group-get?view=graph-rest-1.0&tabs=http.
The article provides a Group creation POST example along with payload https://learn.microsoft.com/en-us/graph/api/group-post-groups?view=graph-rest-1.0&tabs=http
To call Microsoft Graph, the app makes an authorization request by attaching the access token as a Bearer token to the Authorization header in an HTTP request. You can find more information in article https://learn.microsoft.com/en-us/graph/auth/auth-concepts.
Thanks.