Yes you can just prop it open.
That's the way mine is blowing right now.
Just get a dowel small enough to fit through the vent grill. Use a flashlight to see the flap you need to open.
A little force will be needed but not too much. Just go be feel and you should be fine. Place it far enough in the flap so it remains open. Use just enough rod so it does not stick out of the vent.
Easy enough to remove come colder temps. if desired.