Hey, Russell from the marketing team here.
Sorry we didn’t make this clearer, “making Monzo your default card” means saving your Monzo card details or making it the default payment option.
For example, I have my Monzo card saved in Deliveroo, so each time i order, this would count as my default payment option and be an entry into the prize draw 
Also, i’m sorry spend on joint cards is not eligible for this promotion. This is the first promotion of this type we have run and a bit of a test for us. If it’s successful and you all like it, we will run more of them in the future and figure out a way to fix this!