Small plastic dowel/grommet that tilt balancer screws into

I recently ordered 16 spiral tilt balancers to repair my double hung windows that no longer stay open. Everything was going great until I got to the third window and somehow managed to knock out and lose the small square plastic dowel (see photos) that the tilt balancer screws into. Any ideas on where to get a replacement for that piece? If not available could I just screw the balancer directly into the vinyl trim?

Thanks so much!