Welcome to the Magix user to userΒ forums so not a contact for any Magix staff and no-one who posts here can help you, sorry.
If you cannot find the original email confirming your purchase and the information is not to be found in your Magix account under "My products" you will need to email infoservice@magix.net, giving them as much detail as possible about your purchase and ask for their assistance.
I tried again with "Have you registered Melodyne in your Celemony user account? There should be a serial number in your order confirmation e-mail." and found it, its working now.