@Martin468 You get this credit when either you refer a new user to Airbnb or your are referred by a friend. With the referral program the new user has to create their first time new Airbnb account and then go on to book a qualifying reservation.
A qualifying reservation is one where the cost is at least $75 USD (63.70 EUR) before Airbnb's service fee, guest fees and taxes, if applicable. When the referred friend books this type of reservation, Airbnb will automatically apply the Travel Credit.
For the user who initiated the referral, Airbnb will award a Travel Credit once the referred user completes their qualifying reservation.
In a nutshell, that's how it works. There are no other first time user discounts outside of the referral program.
For more details, see these links:
https://www.airbnb.com/referrals/terms_and_conditions
www.airbnb.com/invite/