You were right (of course you knew that)! It turns out my other requests worked because I already had the implicit authorization from my previous coding using PubSub. I never set up Hype Train in that code which is why that one was the only one giving me the 403 error. I added the channel:read:hype_train scope to that part of it (I seem to have misunderstood where I was supposed to define my scopes as well) and now the subscription works perfectly. Thank you again for your help!