How do I make a Caps Lock from a Seach Button?

So, I got a Lenovo Chromebook running the latest ubuntu Mate, and I really hate this Searcch button bringing up the top left corner menu. So, I decided that I will make a Caps Lock from it, but I am new at linux. I already searched the net, but nothing. Changed some the keycode thing, still nothing. As I found out, the key's keycode is 125. Thank you.