Step 1: Press the blue button above. It will take you to the company's Facebook page. There, you will see a post where you can press 'Get Code'. Click that button and you'll be shown a code. Copy it.
Step 2: Go to this Amazon page. Add the product to basket and go to check out. On the final page where it says "promotional code" enter the code you copied in step 1.
Note - if the code doesn't work, go back to the page and request another. If that still doesn't work, write a comment on that Facebook page and the company may give you one.
This post has been updated by Tom.