CE_Iframe.js

function accountNumber(executionContext){
   //retrieve context of current form
   var formContext = executionContext.getFormContext(); 
   //get the value of the accountnumber field
   var accountNumber = formContext.getAttribute("accountnumber").getValue();
   //retrieve the context of the Iframe on the form by it's name
   var IFrame = formContext.getControl("IFRAME_Popdock");
   //get the currently defined url for the Iframe
   var Url = IFrame.getSrc();
   //append the parameter(s) to the base url
   var param1 = Url+"?customer_number="+ accountNumber;
   //assign the updated url to the Iframe
   IFrame.setSrc(param1);
}