Dear readers!

My name is Keifi, and I'm dutch. I just started my website and I'm doing kinda great.
This only one thing I'm stuck on.

I want to make a button that whenever someone clicks on it a ''couponcode '' reveals in the button.
For example like this one.

When you click on '' Get code'' the webshop opens and the code reveals in the button. This is exactly what I want.

How can I make this? Your help is much appreciated!!