Where to put check box javascript

[Mike Grigsby]

I'm trying to automatically fill in a date field when a checkbox is checked, but don't quite get where I would add the following code:

onClick="document.UpdateApplicants_frm.USE__Step1CompleteDate.value=formatDate(new Date(),''M/d/yyyy'')

Do I go to the checkbox field and Client Side tab and pick a local variable that I've equated with this code somehow?