How would I add a cheat code to a title screen

Check for input events (key presses, clicks in certain locations, etc.), and activate the cheat if the correct inputs are received.

Reference:
https://docs.godotengine.org/en/4.2/tutorials/inputs/input_examples.html

If the cheat code is, for example, "iamtheone", you could start a 15-second Timer if an "i" is entered, and then accumulate additional key presses in a buffer until the buffer contains "iamtheone" (activate the cheat) or the Timer finishes (clear the buffer).

Another approach: When a specific sequence of locations are clicked within a time interval, pop up a LineEdit for entry of the cheat code.

Then use input maps.

There's an example here ("First, we need to check for input - is the player pressing a key?"):
https://docs.godotengine.org/en/4.2/getting_started/first_2d_game/03.coding_the_player.html#coding-the-player

Raptorkillz Okay but in the code there's this line

Yeah, but there are other lines as well

Here's an automatic class that listens to action sequences, and when it finds something, it notifies about that via a signal:

class_name CheatCodeListener extends Node
signal code_entered(name)
var s: String
var cheats  = {"ui_up ui_down ui_up" : "some cheat", "ui_left ui_right" : "some other cheat"}

func _input(e):
	for a in InputMap.get_actions().filter(func(aa): return e.is_action_pressed(aa)).slice(0,1):
		s += a if not s else " " + a
		for k in cheats.keys(): if s.contains(k): code_entered.emit(cheats[k]); s = ""

Now the only thing left to do is to make a game to cheat at. Easy peasy

Raptorkillz Okay but how would I make it play a sound

print(String.chr(0x07)). But you'll need to start a console version of Godot for this to work.

trizZzle Konami cheat in a SEGA game? Is this allowed?

I don't think so. But this thread already entered the psychoactive zone, and while passing the threshold it may have created an alternate reality in which this is not only allowed but is in fact - mandatory.