Replacing A Two-Pronged Window Sash Thumb Button

Everything seems to be in order although the new thumb button is slightly smaller, the prongs that fit into the window sash are exactly the same measurement's.

Is there a trick to replacing these without forcing them into the sash and risking breaking them again?

These windows are from 2009, but have no manufacturer on them. I've seen very little on the internet about this type a mechanism with just two plastic prongs. Most seem to have a larger, all in one, unit.