The 403 error indicates you don’t have permission to subscribe to that topic.
While creating an EventSub subscription requires an App Access Token, you still need a user to go through the Implicit, or Auth Code, flow to grant your app the permission, which in this case is channel:read:hype_train
Once a user has gone through your OAuth process and granted your app that permission, you can then use your App token to subscribe to the topic.
Requesting scopes when creating an App token does nothing, as app tokens aren’t associated with a user so at no point does a user agree to let you access whatever those scopes are asking for.