Welcome to Eorzea!

On the main Mogstation page for your account, click on the big 'Manage Service Options' button, then on the next page, you have several options, one of which is 'Add/Change Subscriptions' - this is where you can select a billing cycle from 30, 90 or 180 days, and the type of credit card or debit card you have. Then click Next to enter your card details (card number, CVC number, expiry date etc) and confirm. You can also add time using a prepaid time card that you can purchase from game stores or online shops, by selecting the 'Add Game Time Card' option on the main account page.

I'm not sure how to add Crysta to an account as it's not available in all countries and is automatically disabled as an option if you're accessing it from such a location (it's not available as an option in Australia, which is where I am). So prepaid time cards and credit card/debit card billing is all that is available for me.

I'm sorry I couldn't help with the Crysta option, but hopefully this has helped.