@1_DipikaD 

did you check what came in alert?

I believe your onchange is written on department variable

Are you sure the platform name is correct and the sap choice value is correct? Any spaces in the choice value?

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
   if (isLoading || newValue === '') {
      return;
   }
   var st = g_form.getValue("department");
   if (st == "5d7f17f03710200044e0bfc8bcbe5d43")
   {
   alert('inside if');
    g_form.setValue("platform", 'sap');
   }

   //Type appropriate comment here, and begin script below
   
}

 If my response helped please mark it correct and close the thread so that it benefits future readers.

@1_DipikaD 

Hope you are doing good.

Did my reply answer your question?

If my response helped please mark it correct and close the thread so that it benefits future readers.

Hi @1_DipikaD ,

Can you please try below code?

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
   if (isLoading || newValue == '') {         // change '===' to '==' 
      return;
   }
   if ( g_form.getValue("department") == "5d7f17f03710200044e0bfc8bcbe5d43") // change '===' to '==' 
   {
    g_form.setValue("platform", '8');                                        //set the values
   }

   //Type appropriate comment here, and begin script below
   
}

I correct the script it's working fine.

But drop down value is not changing while I am  changing the reference field value , it remains the same

Can you please check and share the backend name of dropdown field and the value you are setting?