Serial key before my game starts

How do I make a GUI menu for some serial keys that i have choose for example

"1524-OL8D-9NG6"
"G9LK-FHDG-9HG7"
"HUIJ-O17H-E77E"

and when you typed it in it will remember your pc and not ask for codes?
I will need this for beta testers
It can look like this and have a button under it saying:
Unlock

[26987-something+like+this+ingame.jpg|26987]

Maybe you can find your answer in this post - How do I create licenses/product keys for my Unity games? - Unity Answers

As I can read Unity itself uses PACE