You can sign up for Plegium Premium directly through the Plegium app. Simply open the app, navigate to the home screen, and click on the "Sign up" button. From there, you can select the monthly or annual subscription plan and enter your payment information.
Keep in mind that you can also try Plegium Premium for free for one month, and you can cancel your subscription at any time. Subscriptions will automatically renew unless canceled within 24-hours before the end of the current period.