dnd 5e – How do handle grapple contests with “called shots?”

[5e] The RAW rules do not go into any “called shots” when it comes to grappling. Rightfully so. Big difference between 4e and 5e is simplification. However, I am wondering how others are handing grappling contests when it gets very specific. For instance, a fighter wants to grapple a mage and says, “I want to grab the mage from behind and use one free hand to cover the mage’s mouth.” We had this scenario come up and luckily the grappler rolled a nat 20 so the mouth cover was successful. We did not give it much thought. Or, if a character wants to put another in a choke/sleeper hold? How are you handling when there are specific called shots in grappling?