You have multiple ways to get your serial number. First look for the mail you got, when you purchased Music Studio 2019 from Magix. There you will find your serial number. If you purchased Music Studio 2019 from another shop you will have to look for the e-mails you've got from that shop or if you bought a boxed copy you'll have to look inside your box to find your serial number.

If you already registered your copy of Music Studio 2019 you can log in with your e-mail address and password on Magix.com and look up your serial number under your registered products.

If that is all isn't possible you have to contact Magix sales support and ask them for help by providing your purchase information.