When u go pay for your subscription(in mogstation) it will connect to your steam wallet where you can add a maximum amount of money (10€, 25€, 50€ or dollar or w/e is your currency is) to your wallet or you can just pay the exact same amount of money you need to pay. But either way you need to add your credit card number info (name, cc number, valid date etc.). You pay this way too if u buy any other optimal items from mogstation (items, dyes etc.) exception name change, world transfers etc.