Pop! Slots and MGM Slots Live will use two different IDs. If you're connected Facebook or AppleID, your support code will be your unique identifier. If your account is not connected to either service, then it will be your User ID.
In the main lobby, next to the "back" button, there is a gear icon. Tap the gear icon followed by "info". This will bring up a support code and User ID. Most interactions in Pop!Slots will require a User ID instead of a Support Code.