How do I wire this timer enclosure through the wall?

Hello! I bought this enclosure to install a sprinkler timer. It comes with a GFCI outlet, and where I want to install it is next to another GFCI outlet mounted in the wall (outdoors). I thought it would look better to run some wires in the wall, off the GFCI, and into the back of the box so I don't see the power cable on the outside of the house. What is the correct way to run that wire through the wall?