Merging two fields into one field

I have created a document that has first and last name fields to complete. I would like to have the first and last name merged to a 'Full Name' field later in the docuemnt.

Below is the script I tried but was unsuccessfull.

// Get the field values, as strings
var s1 = this.getField("Delegating Physician First Name").valueAsString;
var s2 = this.getField("Delegating Physician Last Name").valueAsString;
// Combine values, separated by a space
event.target.value = s1 + " " + s2;


Please help!!

Denise Cavender