I may be way off here, but I know smaller rolls of Gutermann thread have an insert at the bottom of the spool of thread. You pull in down a bit and wrap the thread around it. That way it doesn't tangle, make a mess, and get wrapped up with your other threads. If your insert that you are referencing in on the bottom, it should have a hole in it though to sit down on the spindle. Not sure without a picture what is is you are referring to, the tab..
There is a step by step guide in the help topic thread that is easy to read and understand. It is what taught this ol' girl how to upload photos, and I consider myself to be very technology challenged.🤪