I have a root site list which I am displaying on several sub-sites using CQWP based on a custom filter and entering the list values frm sub-sites itself to the root site list. 

So I am trying to store the sub-sites url when saving values using default newform.aspx on the list column. I tried storing url value using workflow,event receiver but not able to store as required. I can achieve this using custom web-part but don't want this approach. Using webpart I get the previous visited URL but in event receiver value is null.

Any solutions would be helpful here. 
Thanks!