Welcome to the Magix Sound Forge users community @per-madsen4443,
Log into the Magix Service Center and select "My Products". You must however log-in using the email address you used when Sound Forge (SF) was initially registered and activated. Your Magix forums log-in info may be different.
If you still have SF installed on the old PC, open SF and go to "Help> about Sound Forge", which states your product serial code and the email address (which btw are paired). You may need to deactivate SF on the old PC if it is activated on two PCs already.
Otherwise, email Magix and include your purchase info for verification; sales@magix.net

btw, always archive serial codes and the email address you use for Magix products.