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