get https://e-{networkId}.adzerk.net/udb/{networkId}/sync/i.gif?partnerid=&userId=
This endpoint writes the value of a given RTB Partner's User ID to the user's record in UserDB.
Learn more about user matching here. For nearly every situation, you won't be pinging this endpoint directly - but appending the Kevel pixel to the RTB partner's user matching endpoint.