[Plugin: Magic Fields] give a form checkbox a magic field value

Hello, i’m setting up a search form where i want to allow the users to filter the search via several specific attributes. i’m using wp_dropdown_categories for the category filter. i want to add two checkboxes filtering the search via two magic field attributes, so i’d have something like this:

search: ‘term’ in [category] marked as A [] B []

where ‘A’ and ‘B’ are two magic field attributes

the code, so far:

<form method="get" id="searchform"
action="<?php bloginfo('home'); ?>/">
search:
  <input type="text" value="<?php the_search_query(); ?>" name="s" id="s" />
 in <?php wp_dropdown_categories( $args ); ?>
<input type="checkbox" name="A" value="" />
 <input type="checkbox" name="B" value="" />
<input type="submit" value="→" id="searchsubmit" />
</form>

thank you, hope it was clear