Help with Acrobat form javascript

I need help with pdf scripting. I have a Acrobat Pro document with a template called "Dupe".

There are two fields: MM.0 (a drop down menu) and RT.0 (a text flied). I want to be able to have the user be able to select more than one option from MM.0 and have the selections appear in RT.0 separated by "." and a space (As in, different sentences.) The following script in Validation for MM.0 works on the template page, but not when the page is spawned:

//function PreFix();

var cPreFix="";

//get prefix elements for spawned pages;
var aFieldName=event.target.name.split(".");
if(aFieldName.length > 2)

{
cPreFix = aFieldName[0] + "." + aFieldName[1] + ".";
}

//To permit multiple selections from MM;

UpdateTextField(this, cPreFix + "RT.0", event.value);