Manufacturers instruct users to add items
What r the manufacturers instructions for adding an item in Find Me app?
Windows, Windows 10
If it is an Apple AirTag, it's clearly indicated in the support article that led you here.
If it is a 3rd party item, if you already tried (to no avail) the steps provided at the section "Add a compatible third-party product" in the same article, I suggest that you contact your item's manufacturer support team and ask them.
Regards
Giulio