Call the Users endpoint
Without specifying a userID or login.
A user is also a broadcaster, different labels are used depending on the endpoint being discussed
The example from the documenation is
curl -H 'Client-ID: yourClientID' \
-H 'Authorization: Bearer cfabdegwdoklmawdzdo98xt2fo512y' \
-X GET 'https://api.twitch.tv/helix/users?id=44322889'
But if you do
curl -H 'Client-ID: yourClientID' \
-H 'Authorization: Bearer cfabdegwdoklmawdzdo98xt2fo512y' \
-X GET 'https://api.twitch.tv/helix/users'
You’ll get the user JSON for that Token and can extract the ID from that.
Edit: note the examples are invalid due to the Helix API changes and I just went and lifted then from the docs
Edit 2: fixed the calls