I’ve linked to the fly for free website but basically book by 27/4/20, stay by 31/3/21 and get 15€ off. (Edit I tried to, but they aren’t allowed for some reason)
I started a booking for bali and the code was accepted but for a smaller amount. You still have to pay taxes if the hotel costs 15€ or less.
Link to fly for free web page
https://www.fly4free.com/flight-deals/europe/hotels-de-promo…
Don't book anything unless you want to be an unsecured creditors these days