If you look at the wording for Moonbeam, it says "A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high cylinder centered on a point within range." It doesn't say anything about having to "target" the point, you certainly don't have to see the point, and the effect does not originate from the caster.Edit: though thinking more about it, I don't think you can summon inside a Wall of Force once it is up. It counts as "total cover," and the rules on total cover are clear: unless the spell description specifically stipulates otherwise, you cannot cast a spell targeting something behind total cover. In this case, they would be trying to target a summoning spell behind total cover. It would be like trying to cast it into a closed building from outside.