Link_to popup pass parameter

hi, im poping up a window with:

<%= link_to(“Add new Item”, “/bol_lines/new”, :popup => [‘window_name’,
‘width=600,height=600’]) %>

now, that link resides in teh edit view of the parent of the 1<>N
asscoation. the child window is supposed to be the NEW view of the
child-association. but to submit the new view containting the
bol_lines form i need to have the parent_id as hidden field or
how can i achieve that?