Good replacement for selected?

I have it so if a player presses / and then types something in that text that they typed in would go in a text box. It's like the roblox chatting, except I want it to be custom. Can anybody point me in the right direction?

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

mouse.KeyUp:connect(function(key)
    if key:byte() == 47 then
        script.Parent.ChatBox.Selected = true --Selected doesn't work. What will?
    end
end)