You can sign up for GoPro Plus from the following apps
1. GoPro App (iOS and Android)
Either from the ‘CLOUD’ tab (i) or from the settings menu (ii)
3. Quik for desktop
a. Settings > Cloud
b. Add Premium Soundtrack
*Signing up through Amazon requires you to then link your GoPro account to the subscription. For further information on how to link your account, take a look at the page here.