The grant type for generating App Access tokens, is literally client_credentials. That’s the string that must be in the url, you don’t need to replace it with anything.
The docs example show how it should look POST https://id.twitch.tv/oauth2/token?client_id=uo6dggojyb8d6soh92zknwmi5ej1q2&client_secret=nyo51xcdrerl8z9m56w9w6wg&grant_type=client_credentials
The client_id and client_secret are from your app on the TwitchDev console, but the grant_type is simply the string client_credentials