Field calculation not being run as expected.

I have the following script.  This script concatenation the first and last name into one field. When i run the application the first and last name are filled in.  The new field shows up blanks.  After opening the calculated function and closing it the data appears in the new field.  I need the calculated field to appear when I run the application.

var s1 = this.getField("insured.fields.first_name").valueAsString;

var s2 = this.getField("insured.fields.last_name").valueAsString;

event.value = s1 + " " + s2;

Message was edited by: Malcolm Thomson, updating title.