If you happen to have a special link from a friend referring to a promo code page, then it's applied already. It requires to just follow the link.

 If you have a promo code in a written form, you can fill it in on “Pricing” page. 

  1. Go to the Pricing page.
  2. Scroll the page until you see  "Have a promo code from a friend? Redeem it" 
  3. Type your promo code and click "Get free credits"

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