In NetSuite, the beforeLoad user event script is used to perform actions before a record is loaded. The custom form id in view mode using the entry point beforeLoad may not be able to retrive. But it is available in beforeLoad in the edit mode. It wont be available from scriptContext. form.
So, to get the id either we have to load the record in the beforeload dynamically and retrive the id or make use of the search.lookupfields in the beforeLoad to retrive the id.
Since, loading the record in the beforeLoad is not a good practice, search.lookupfields is the possible solution.