How can i limit the search results on the current forum?

I forgot the code tags, sorry

I want to change the search-block in order to search only inside the current forum and not the entire forum.

The search-block code is:
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<form action="/drupal-test/"  accept-charset="UTF-8" method="post" id="search-theme-form-1">
 <input type="text" maxlength="128" name="search_theme_form" id="edit-search-theme-form-1" size="15" value="" title="Search" class="form-text" />
<input type="submit" name="op" id="edit-submit-3" value="Search"  class="form-submit" />
<input type="hidden" name="form_build_id" id="form-2def14ad6cb5832ec031c45ba0841b36" value="form-2def14ad6cb5832ec031c45ba0841b36"  />
<input type="hidden" name="form_id" id="edit-search-theme-form" value="search_theme_form"  />
</form>

The advanced search uses this field to set the forum:
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 <select name="category[]" multiple="multiple"  class="form-select" id="edit-category"  size="10">..</select>

I want to make this a hidden field. Any ideas how can i do that ?

thanks