MuseScore Pro account is $49 USD per year or $6.99 USD per month. A non-trial annual Pro subscription is $29,99 USD.
So you can choose which option is better for you and add, change or edit your subscription details at:
One Pro subscription is valid both on the web and in the Mobile App.
To access your Pro account simply close the windows that suggest starting a trial in the App and log in using your username and password.