Normally you would set the redirectURL to a server you control.
And on that server you put your code to run/create/perform validation.
If you are not planning on using Authentication with twitch then you do not need to put a redirectURL, just need to put http://localhost/
What are you trying to do? If you are just trying to obtain a ClientID in order to use the API, set your redirectURL to http://localhost/ and ignore it