Determine the Base User Exit
For this example, we want to disable the input element corresponding to the start date and display a message that the start date is disabled. This means we want to disable the field when the page loads; and therefore we want to insert our code inside the onWindowLoad() function. The external user exit function that allows us to do this is the extPostOnWindowLoad() function.
You can check the field names under the payPlanMaintPlanPage's Labels section.

    * Program name:      payPlanMaintPlanPage
    * Program location:  /ci/payPlan
    * Program version:   68
    * Program template:  UIXTP
    * Template file:     //FW/2.2.0/Code/modules/web/source/root/WEB-INF/uiXSL/tabPage2.xsl 
    * Template revision: 4
    * Included XSL versions: 
    *   common           
    *   commonPage       
    *   commonPageSingleRecord 3
    * 
    * Labels:
    *     Table$Field  -  label   (element type, js_name) 
    *     $PP_LBL  -  Pay Plan   (element type='L' , jsName='PP_LBL') 
    *     ...
    * Widget Info:  
    *     Widget_ID , Element Type - label info - label 
    *     ...
    *     START_DT, IT - $START_DT -  Start Date

    *     PAY_METH_CD, IS - $PAY_METH_CD -  Pay Method
    *     ... 

The two important pieces of information in this source view are:
The Program name definition - payPlanMainPlanPage in this example;
The Template file definition - tabPage2.xsl in this example.