Vygotsky There should be a Shortcuts app on your homescreen. You can create a new shortcut there and then click on "Add action", choose "App", then choose "bunq", and you'll find some actions you can add take to your shortcut, like changing the account that your card is associated with. Then as the next action I would go to Scripting, choose "Open an app", and then go for the "Wallet" app for example.
As a last step you can give the shortcut a name like "pay from my groceries account", and when you save it you can just tell Siri this exact phrase and the shortcut will be executed.
Edit: Ok that tutorial is probably better. Didn't know about it.