I have an application created on Twitch, the issue is that I need it to work on multiple sub-domains. Am I able to use a wildcard (*) in my return URL for the application? Otherwise I’m going to have to create potentially hundreds (or thousands) of separate applications on Twitch by hand.
Twitch doesn’t support multiple callback endpoints, so you will have to register multiple different apps in the meantime to get this functionality or create a callback handler to redirect users on your site. You may also be able to do some creative things with the OAuth state parameter.
You should make a single callback page that redirects the user back to the subdomain they came from. You can either store the desired subdomain in the user’s session, or by putting it in the state parameter.