How to add a text cap/limit to a text import?

Since the Changed event method didn't work, we would have to detect when the client presses a key on the keyboard. We could do this using InputBegan, an event from UserInputService:

local UIS = game:GetService("UserInputService")

local TextButton = script.Parent

local MaxCharacters = 50
local InputBeganConnection = nil

local function DisconnectInputBeganEvent()
	-- since events can be connected, you can cancel it by disconnecting
	-- it is important in this case since we reconnect the event when the TextButton gains focus.
	-- it'll stop events from stacking and allows us to cancel them when they're unneeded
	if InputBeganConnection then 
		InputBeganConnection:Disconnect()
		InputBeganConnection = nil
	end
end

TextButton.FocusLost:Connect(DisconnectInputBeganEvent) -- FocusLost fires when the TextButton loses selection

local function OnFocused()
	DisconnectInputBeganEvent()
	InputBeganConnection = UIS.InputBegan:Connect(function(input)
		if input.UserInputType == Enum.UserInputType.Keyboard then 
			TextButton.Text = string.sub(TextButton.Text, 1, MaxCharacters)
			-- string.sub has 3 arguments: s(string), i, j
			-- it returns the substring of s that starts at i and continues until j
			-- for example: string.sub("Text", 3, 4) would return "xt" because
			-- 'x' is the third string and 't' is the fourth
		end
	end)
end

TextButton.Focused:Connect(OnFocused) -- the Focused event fires when the TextButton gains selection (when the user clicks the button)