Here's a step by step for you:
Go on Amazon and look for 'Arlington TVL508-1'. That is the box you need to use.
Cut the power to the box that you are going to tap into. Move the wires out of the way and drill a hole in the top of the box, near the front and to one side, large enough for the romex to get through. Make sure the hole is not in the center of the box
Cut the opening for the new box that you are going to put behind the tv. Be sure to dry fit the box but don't install it yet.
Use a fish tape if you have one and work it up through the hole you made in the lower box, until you can see it in the opening you made for the upper box. There should be a gap between the drywall and hard foam insulation where you can pass the fish tape.
Attach the romex to the fish tape and pull it down until it feeds into the lower box.
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