On Android you can search in help articles add credit card, and at the end of the article there’s a direct button to add cards, check if there’s the same on iOS.
From the help pages
We hope to support as many credit card providers as we can.
Currently we’re using a service called TrueLayer to connect to credit card providers, and can only support the banks they work with.
By September 2019 credit card providers will need to offer a way for apps like Monzo to connect to them. As each provider makes it possible to connect, we’ll work to add them to the app!
I believe the following 3 things will need to happen:
Capital One will be need to release an open banking API (this may or may not have already happened)
Truelayer will need to support Capital One (this has not yet happened). Truelayer is a company Monzo is using to facilitate connections to credit cards.
Monzo will need to decide to support Capital One via Truelayer.
When it gets reworked in the next couple of months to handle the API feeds that will hopefully then start to be released by the banks etc, a partial release as we have now will be easier to migrate than one for 2.5m people.
Once the new version is developed, that will be pushed out to all and sundry.
Heaven only knows what list of banks/cards that will support though!
(Support will depend on the card issuer releasing an API and the aggregator integrating it into their software before Monzo/we get visibility of it.)