The spell description on page 288 of the PHB says this: "You conjure a mass of thick, sticky webbing at a point of your choice within range. The webs fill a 20-foot cube from that point for the duration. The webs are difficult terrain and lightly obscure the area. If the webs aren't anchored between two solid masses (such as walls or trees) or layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn."

That would imply to me that you have to use it on an area, and the creature has to go into that area. However, it also says this: "Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is restrained as long as it remains in the webs or until it breaks free."

Based on that, I would personally be fine with the player wrapping it around the creature (as if they were layering it onto a surface), but I would probably require an attack roll in order to succeed, and then it would fall out of the sky, since restrained prevents them from moving.

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