An in-app selection would be a simple solution for now, at least for the purpose of testing the popularity and usability of feature.
I doubt an automatic selection would always be appropriate though. Imagine you regularly shop somewhere, a supermarket for example. You regularly pay for weekly shop out of the Bill Splitting account and your partner hints at you that they’re interested in something expensive they’ve seen at the same shop for their upcoming birthday or anniversary. You wouldn’t want it automatically get selected to come from the usual Bill Splitting account. Instead, I would prefer to be able to select that specific purchase come from a Virtual Savings Pot named “Gifts”. That way the partner would be left unaware that you’ve bought them that gift and of course it would then be 100% gift rather than some money coming from the person that you’re buying for.
I do love the idea of having a different card per subscription service though. Absolutely brilliant idea. Then you could simply cancel/freeze that virtual card if you get into a dispute with that service. That’s getting off-topic, but great idea nonetheless.