"To add the mint" means to include or put in some fresh mint leaves or extract to enhance the taste, flavor, or aroma of a dish or drink. Full definition
Just add some mint leaves and a few chocolate chips! (aggieskitchen.com)
And instead of just going with a plain chocolate ice cream (which you could also do), I decided to add mint extract and some cacao nibs for a little crunch. (thehealthyfamilyandhome.com)
Get inspired and add some mint in your tea and in your house! (blog.netrobe.com)