You can use the Get User Follows endpoint https://dev.twitch.tv/docs/api/reference/#get-users-follows and use the from_id param.
If you don’t know how to get the ID from a username, use the Get Users endpoint https://dev.twitch.tv/docs/api/reference/#get-users and specific their username as the login param. In the response will be their Twitch ID which you can use with that Get User Follows endpoint.