Mounting TV on Plywood over Window
You do not want to cover over the window with a TV. First of all there is the glare point that  Paulster2 points out. That alone should make you take stock of this situation and see what a bad idea this is. However there are a number of additional considerations as well.


Windows have a role of safety exit possibility if you ever get caught trapped in a room during a fire. This for both outbound escape and for inbound access by emergency personnel. 
There is a reason windows are added to a house. Obviously it is to allow light in so that the room does not look like a cave. 


If you have no usable wall space to mount the TV then get your self a console table. These do not have to be that expensive and you can even get them with an integrated post assembly that permits mounting the TV bracket with tilt and swivel capability. Best with this approach is you can move it around if your change your TV room layout or go off to a different house. 

Talking about moving. Say you did move after you had mounted a TV over a window. Think of the mess in the window trim that would need repairing before you could sell. 

Lastly, putting a TV over a window is just weird.