not sure this is possible directly with siri shortcuts, but maybe through the bunq api? I think that it's possible to assign cards to accounts in the third party bunq desktop application from @Gregory so it should also be possible through the API. but I don't think bunq app exposes the api for local, on-device use. but this could be put on the wishlist.
UPDATE 10 July 2019: it is now possible thanks to bunq update 11 from yesterday! Please check out this topic from bunq employee @Andreas to find out how to set it up.
But remember this is currently iOS only (as it requires Siri / Siri Shortcuts) and you need at least iOS 12.2 or higher and the latest bunq app v11.0.0. Happy bunqing 😃