How do I silence a guard and not kill them?

Building off of the below thread:  https://www.dndbeyond.com/forums/d-d-beyond-general/general-discussion/237728-grapple-covering-enemys-mouth

I pass my stealth et. al. rolls and I am behind a humanoid "guard".    I have no magic capability.   How do I silence this humanoid so that he does not verbally cry out.

It appears per the linked thread, grapple may not be correct or if it is, it is unclear how that would work.   Because grapple may not be correct, I started a new thread so ther is no bias in what action should occur.

In a non dnd game, I would reach around and cover the mouth.   

How would I achieve the same result (not necessarily mirror the action) in a DnD game?

ask the GM

Thank you, that is a given, on what is allowed, but still want to know how.

The "how" will be decided, ad hoc, by the DM because there aren't any rules for this. That's literally why the other thread was made.

Thank you all.

An improvised action is up to DM.

Actions

Player characters and monsters can also do things not covered by these actions. Many class features and other abilities provide additional action options, and you can improvise other actions. When you describe an action not detailed elsewhere in the rules, the Dungeon Master tells you whether that action is possible and what kind of D20 Test you need to make, if any.

A DM can look at Rope for inspiration to gag or muzzle someone and if the creature’s mouth is bound, the creature can’t speak until it escapes for example.

Rope (1 GP)

As a Utilize action, you can tie a knot with Rope if you succeed on a DC 10 Dexterity (Sleight of Hand) check. The Rope can be burst with a successful DC 20 Strength (Athletics) check.

You can bind an unwilling creature with the Rope only if the creature has the Grappled, Incapacitated, or Restrained condition. If the creature’s legs are bound, the creature has the Restrained condition until it escapes. Escaping the Rope requires the creature to make a successful DC 15 Dexterity (Acrobatics) check as an action.

Can you still choose to make a melee attack knock a target unconscious in the 2024 rules?  The tap on the head is a classic.

Yes you still can;

Knocking Out a Creature

When you would reduce a creature to 0 Hit Points with a melee attack, you can instead reduce the creature to 1 Hit Point and give it the Unconscious condition. It then starts a Short Rest, at the end of which that condition ends on it. The condition ends early if the creature regains any Hit Points or if someone takes an action to administer first aid to it, making a successful DC 10 Wisdom (Medicine) check.