@Debarpita Basak 

I already mentioned the another approach i.e. using Display BR and the UI Action

Did you try that?

Also you can use this if you want scratchpad variable

(function executeRule(current, previous /*null when async*/) {

	// Add your code here
	var ritms = current.u_number.toString();
	var arr = [];
	var gr = new GlideRecord("sc_req_item");
	gr.addQuery("sys_id", "IN", ritms);
	gr.query();
	while(gr.next()) {
		arr.push(gr.request.requested_for.toString());
	}
	g_scratchpad.users = arr.toString();

})(current, previous);

Then use onLoad client script to set the value

function onLoad(){
	if(g_scratchpad.users != ''){
		g_form.setValue('u_to_whom', g_scratchpad.users.toString());
	}
}

Regards
Ankur