The way I’d approach this is to stop struggling with Monzo and, instead, go to the credit card (website / telephone support) and pay using debit card with the Monzo card details.
I appreciate that doesn’t answer your query about the missing reference field, but it does enable you to pay your credit card.
The payment flow and references is something I brought up with Monzo back in 2016. Both Monzo and Starling had payment flows inferior to some of the legacy banks. Clear that it’s not been a priority for them.
Its my original bank accounts credit card so it’s easy enough to pay direct from there but since I use Monzo as my main account I’m trying to avoid the need to transfer to my other account and then to my credit card.
Since the option was there last time I paid I kind of assumed I was missing something.