How can make validate on field before workflow action?

ERPNext Customize ERPNext
,
1

how can make validate on field ( agency_name) for the below code

i need user put agency name before he choosing workflow action (Send to agency’)

frappe.ui.form.on('D Maintenance', {
    	
    before_workflow_action: (frm) => {
        if ((frm.selected_workflow_action === 'Send to agency')) 
        
        
        {
           
            
          frm.set_value("date_of_receipt_from_agency", frappe.datetime.now_datetime());
            frm.refresh();
            frm.save(); 
            
        }
        

}});