If you download and install the Trial Version you will need to request a Trial Key via a link in the registration process, which you will need to complete when you attempt to run the app for the first time.. You will then be emailed a key, which you can use to run a restricted version of the software for, I think, 30 days.
If you don't request a Trial Key during registration the software will not work at all.
When you sign up for the (try for free) trial version of Sound Forge , Magix/Vegas emails you an activation serial code which you can use for 30 days without limitations, aside from the included third-party add-ons. If you go ahead and purchase, you get a permanent serial code and you do not have to uninstall/reinstall the app. just enter the 'new' serial which is valid forever. The '365' subscription version activation serial is only valid when the subscription is paid up. I am not sure where there is a trial version of the lower cost Sound Forge Audio Studio 17.. there used to be. SFAS is currently on sale for $35 usd. There is no Audio Studio version of SF-18.
btw, welcome to the Magix Sound Forge users community @karaflas.