If you start Sound Forge for the first time you need to enter an e-mail address (this is your Magix.com user account e-mail) and a serial number. After that your product is registered to your user account.
And... make sure you save the serial number and the email account used to register the product. It is not uncommon for folks to forget the email account that was used, and then have trouble not being able to download sometime in the future.
btw, welcome to the Magix Sound Forge users forum @Louis-Mercieaa.