Wrong tool for the job. That tool was meant for drywall. It does come with a 1/4" collet, so you could put a 1/4" router bit in there. There are two caveats:
1. A bearing guided bit will have the bearing on the bottom, so you'll need to drill a pilot hole, not a big deal. This tool was meant to feed into drywall by tilting the base at 45 degrees to the drywall and slowly raising the router until the base is flush with the surface. I would NOT recommend doing that with plywood.
2. Rule of thumb with routers is not to exceed the width of the bit in cutting depth. You can stretch that a little, but at some point it puts too much stress on that bit and it snaps (at 27K rpms). 1/2" ply you might be able to do going slowly. 3/4" ply expect the bit to break. Regardless, if you're cutting a long length for a window or door, you're really using the wrong tool, get a saw.