The broadcaster_id is the userID of the channel you want to look at
The moderator_id is the userID of the moderator in the oAuth token you are using.
Username/login to ID lookup can be done via the Get Users Endpoint
Or you can use the token with get users with no params to return the user for that token.
So if I want to get the chantters in cohhcarnage, using my token
broadcaster_id = 26610234
moderator_id = 15185913
using my token