My understanding is that apps purchased on the Microsoft Store are installed and authorised/activated on a computer via your Microsoft account and it is your Microsoft account that recognises the purchase of that app from the Microsoft Store. If you've installed Vegas Pro via your Microsoft account or the Microsoft Store (i.e. not from MAGIX's website) and it is that installation asking for product key, you will need to contact Microsoft because Microsoft Store editions are versions specifically tailored for their store (e.g. Screen Capture and some rendering codecs are not included In Vegas Pro 18 Edit Windows Store Edition [VP19 not currently showing]).
I do not know if this applies to 'Vegas Creative Software', but when a downloadable version or Sound Forge is purchased from third-party seller, like Best Buy, Musician's Friend, Guitar Center, Amazon, ect,, the buyer receives a voucher to redeem at Magix, which emails the actual activation serial code. See Magix's. 'How can I redeem my voucher', As I stated, I am not sure this applies to Vegas or the Microsoft store.