Making a Lookup field in Subform as Read only through Client Script

Making a Lookup field in Subform as Read only through Client Script

Hi
I want to know how can i make a lookup field(ookup to Products) in a subform on Deals module as Read Only on a Stage through Client Script in Zoho CRM. I was able to make the other fields as Read only but was finding issue with the Lookup field. Can anyone help me here.