How do I adjudicate a player trying to sweep kick an enemy’s legs to knock them down?

I’m new to D&D. I already read both the Player’s Handbook and Dungeon Master’s Guide (I’m mastering) and I would like to know how I could handle this situation:

A player wants to do a sweep kick on an enemy’s legs and knock him down. He rolls a 20 and does the damage, but what about taking the enemy down? There is no rule that covers it.

Following the rules, the player would have to do a specific action to try to push the enemy down, and there would have to be a resisted check to see if the player can actually drop the enemy, so how could I deal with that?