No, not for Soco alone: Apple manages payment methods at the Apple Account level, so one card can't be tied to a single subscription. What you can do is put the card you want at the top of your Apple payment list, because Apple charges the cards on your account from the top down.
Make a specific card get charged first
Add the card you want to your Apple Account, then move it to the top of the list on your iPhone:
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.
Check your chosen card now sits first.
To add a new card before reordering, tap Add Payment Method on the same screen. Apple's guide: Add a payment method to your Apple Account.
Why there's no per-subscription card
Soco has no setting for it, and Apple manages cards at the account level: the payment list on your Apple Account covers every subscription and App Store purchase together. The order of that list is the only control you have, and it applies to everything billed through Apple, not just Soco.
Next steps
Your next question is probably one of these: