Where to add a "What's This?" link

I created a custom field for customers to fill out when registering on our website. It’s a field for their unique customer ID that’s located on the customer loyalty cards we give out when they sign up at our physical location.

Which file do I edit and where in that file do I go to add the code for a “What’s this?” link next to the field so people can click on it and get a pop-up box that clarifies what that field is asking for?