You need to follow the Authentication docs https://dev.twitch.tv/docs/authentication/
There are several methods to obtain a token, which one you use is up to you and depends on your use case. Also keep in mind that you need a token of the channel you wish to receive events from. So if it’s your channel you can go through the auth process yourself, if it’s for a channel belonging to someone else then they will need to go through your authentication process.