first, find a random object, (dont make it to small and make sure it has collision) and put it next to this wall. for me, i chose this pallet:
then, stand on your prop:
...and now, prop climb! (to prop climb, just spam the use and jump at the same time in rapid succession) do this until you reach the building.
you could also prop climb here:
...but theres this weird invisible clip brush in the way, and can be quite annoying to deal with.

anyway, once your over there, just jump down here: