The ReferenceLookupWindowField Formulator widget does not accept external links.
If "http://server/path" is entered by hand into the text input field of the
widget, the "http://server" part is deleted when updating the a code source
containing such a widget. This behavior is different from the link input field's
behavior. In my opinion the "http://server" part should not be deleted in order
to allow internal and external references.