display comment form for specific post id

Use get_comments() and pass the post ID as parameter:

$comments = get_comments(
    array (
        'post_id' => 343
    ) 
);

foreach ( $comments as $comment )
{
    // Just to give you an idea of the available data.
    // You will probably do something better with it. :)
    var_export( $comment );
}

Related: Bug #20572 ($post_id not passed to comment_open() from comment_form()) was fixed just two days ago.