Adding mint extract to french vanilla creamer

From afar, I don't know the strength of your mint extract, but usually they are very strong and sometimes artificial tasting. To balance the strength and avoid the artificial possibly dominating, start with the smallest amount you can add (probably 1/8 tsp), leave it in the fridge overnight to disperse and then taste.
From that taste, you'll get both an idea of how strong the extract works in the the creamer and how you both like it. Then add more in units you feel comfortable with, always keeping it small, because it's easy to overseason but nearly impossible to remove too much mint.
Make notes when you get to a flavor level you and your wife like, so you can match it the next time you make it.
Last, if this is an important treat or indulgence for her or you both, it is worthwhile to find a high quality, non-artificial mint extract to use.