How you add codes to a Tycoon game?

First off you need to add a gui with a textbox on your screen, assuming that you got that, all you need to do is make a text button next to it and add this script into it:

script.Parent.MouseButton1Click:connect(function()
    if script.Parent.Text = (whatever you want your code to be for that time) then

        --For the rest of this you simply need to give them the rewards that you want.

    end
end)

If you had numerous codes at one times your script could also look like this:

--// Variables
Codes = {}

--// Redeeming

script.Parent.MouseButton1Click:connect(function()
    for i, v in pairs(Codes)  do
        if Codes[i] == script.Parent.Text then --[[since i changes to the amount of times we                
                                      run, it will start at 1 and keep going, this means we can check each 
                                      value in the table, from one to however many we have and see if what they typed is a code.
    ]]
    --For the rest of this you, again, simply need to give them the rewards that you want.
        end
    end
end)

Hope I helped and please accept my answer if I did.