How to order search results so a specific page appears first when user searches for a particular word?

Question

You need to be reading the search parameter from the URL. By default WordPress uses the parameter s to store the search results. So you’d need to do something like:

$user_search = ( isset( $_REQUEST['s'] ) ? '*' . wp_unslash( trim( $_REQUEST['s'] ) ) . '*' : false );

if ( false != $user_search ) {
    $query->set('search', $user_search);
    $query->set('search_columns', array( <comma separated list of any fields other than content you want searched> ));
}