Just take a peek at the page source for advanced search. That's all I do. I notice these checkboxes:
Code:
<TD width="5"><INPUT type="checkbox" name="posted_data[category_main]"></TD><TD nowrap>Main category</TD>
<TD width="5"><INPUT type="checkbox" name="posted_data[category_extra]"></TD><TD nowrap>Additional category</TD>
so if I wanted to make sure "additional categories" is checked in the auto search then I would just add the following:
Code:
<input type="hidden" name="posted_data[category_extra]" value="yes" />
You can see how "category_extra" is referenced in
include/search.php. You can add most of the advanced search fields in this way. You could also pass in another variable in your GET string to turn this on/off.