The Application Name will be displayed to users when they attempt to register via GitHub.
The Homepage URL should be set to the Board URL value set in XenForo.
The Description is optional.
The Authorization callback URL should be set to <XF board URL>/connected_account.php. For example, https://xenforo.com/community/connected_account.php. The beginning of this URL must match your Board URL setting in XenForo exactly.
Once you click Register Application note down the Client ID and Client Secret.