Hi Fran,
Short answer - you can’t. Because IFTTT/Monzo don’t support Monzo Joint accounts (yet)
But I do have both Monzo Personal account transactions (not really used) and my/our Joint account transactions (used too much) automatically pushed to my YNAB set-up - but not using IFTTT
It’s not particularly easy, but possible, using the brilliant instructions kindly posted by @BritishLibrary - Automatically import transactions to YNAB
Works really well.
And as @Ordog points out, some recent work on Joint account integration with IFTTT may eventually result in a simpler way to interact with Monzo & YNAB moving forward 