Call Get Users and omit the ID and login
Gets information about one or more specified Twitch users. Users are identified by optional user IDs and/or login name. If neither a user ID nor a login name is specified, the user is looked up by Bearer token.
So using the example
curl -X GET 'https://api.twitch.tv/helix/users' \
-H 'Authorization: Bearer cfabdegwdoklmawdzdo98xt2fo512y' \
-H 'Client-Id: uo6dggojyb8d6soh92zknwmi5ej1q2'
Will return the user whom belongs to that oAuth token. (Example from the documentation)