Hello @Roland126,
If you have travel credit it should appear as a deduction on the payment page when you're about to pay for a trip.
If you were given a coupon, as happens when a host keeps superhost status for a year, then there should be a button or link on the payment page saying something like "use coupon code". That's used to enter the code given in the email notification sent when the coupon was awarded.
Trips have to be of a minimum value to qualify for use of credits. It's usually the local currency equivalent of US$ 75 or so excluding taxes and Airbnb fees. Coupons can be used even if the trip cost is less than that.
Steve.