I just found the website: www.csgodouble.com

1. Go on the site and register with ur steam account (trust me its not a scam)

2. Click on Free Coins

3. Enter the redeem code: TERRORFX
You will get for this code 500 FREE coins, and yes it is also a ref-code

4. And now you can ez multiply your Coins

5. If you have enough coins you can buy skins on the website

Enjoy!