When u have glasses (brille) with a tight strap at the back of your neck, u can use it to keep your glasses on during the ride.
(Copy of the email i got from Phantasialand on September 18 2020 :)
thank you for your request.
We can to tell you that you can only wear your glasses at our newest attraction F.L.Y. with a glasses strap due to safety reasons. Because it can cause severe turbulences, all loose objects have to be stored in the special lockers in advance of your flight.
We wish you a pleasent flight and a lot of fun at your destination Phantasialand.
Best regards from Phantasialand