Yup, you just need a filter.
So, on your rule, your value is currently User_Name. Click the fx box to open the formula editor again. Highlight the User_Name field and select Insert Field or Group. IP should open up a data source window with the field selected. In the lower left corner, you'll see a filter data button. Click that.
Now you'll need to add your filter... How will you know which field is the correct one? I'm assuming there is some other data in the form that matches something in your list to let you know which user name you need?