Custom Field Search

Use pre_get_posts to filter search results:

function search_filter($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_search) {
      $query->set('post_type', 'custom_post_type_name');
      $query->set('cat', intval($_POST['cat']); // assuming you have a select with categories with name "cat" 
    }
  }
}

add_action('pre_get_posts','search_filter');