How can I run electricity to a shed from an outlet on the external wall of the house?
If you want to tap that circuit, get a box extension that has 1/2” conduit entrance. Most locations will require a “bell box” extension something like a 5400-0 . This brings the existing box out so you can connect conduit to the box and bury it and come up in the office.
You will need the conduit from the box down the wall into the ground to burial depth. Depending on the distance and work to bury rigid conduit may be best, only need to bury 6”, if you use pvc 18”, or go with underground feeder / direct burial. If using conduit you will need a thhn/thwn wire type almost all thwn wire is dual rated thwn.  With a 15 amp circuit 14 awg wire is needed.
There are other ways to tap into circuits but a box extension at an existing location is the easiest. If this is the circuit you are using now you know it will hold for the loads because you are using it now. You will need GFCI protection on the receptacles.