Have my serial number for fl studio mobile but forgot my apple Id how can I download it

I have my serial number for fl studio mobile it's stored on my image line account which is fl studio mobile creators of the iOS app I don't have my apple Id don't remember it at all so how can I get back on my device I have it registered on the FruityLoops website they are telling me to contact you guys to get my app back since i paid 19.99 for it