Apple manages payment methods at the Apple Account level, so you can't assign one card to only your Soco subscription. You can, though, reorder your cards so the one you want is charged first.
Why you can't pick a card for just Soco
You can't pick one card for only your Soco subscription because Apple charges the cards on your Apple Account in the order they appear, across every subscription and App Store purchase. There is no setting, in Soco or in Apple, to tie one card to a single subscription.
How to make a specific card get charged first
To get a specific card charged first, add it to your Apple Account, then move it to the top of your payment list:
Open the Settings app and tap your name
Tap Payment & Shipping
Tap Edit
Touch and hold a card, then drag it to the top of the list
Apple charges the cards on your account from the top down, so the card at the top is used first. To add a new card before reordering, tap Add Payment Method on the same screen. Apple's guide is here: Add a payment method to your Apple Account.