Connect the garden hose to a piece of PVC conduit and you can wash it under the sidewalk. Dig a hole on each side and go for it. Then run PVC from the main panel to the new receptacle box. Be sure to include a ground wire in the run.
If you need to make a small bend in the PVC here and there, heat it up with a heat gun or propane torch (slowly) or whatever. I use an electric oven on short lengths to make custom offsets and the likes.
You'll be providing GFCI protection of course, right?
Have you figured how to get out of the main panel to the outside?
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