puedes usar esto, pero solo servirá para cuando estes dentro de un tema, de cualquier foro, no es posible hacer el codigo para que aparezca en la portada por la forma en que esta escrito SMF.
echo '
<a name="quickreply"></a>
<div class="roundframe windowbg2" style="position: fixed; bottom: 0px; left: 1%; width: 98%; text-align: center;">
<div class="quickreplybar" ', $options['display_quick_reply'] == 2 ? '' : ' style="display: none"', '>
<div class="floatleft" style="padding: 0 3px 0 3px; width: 10%;">
<strong>', $txt['quick_reply'], '</strong></div>', $context['can_reply_approved'] ? '' : '<em>' . $txt['wait_for_approval'] . '</em>', '
', !$context['can_reply_approved'] && $context['require_verification'] ? ' ' : '', '
', $context['require_verification'] ? '<span class="smalltext">' . $context['require_verification'] . '</span>' : '', '
<form action="', $scripturl, '?action=post2" method="post" accept-charset="', $context['character_set'], '" name="postmodify" onsubmit="submitonce(this);" style="margin: 0;">
<input type="hidden" name="topic" value="', $context['current_topic'], '" />
<input type="hidden" name="subject" value="', $context['response_prefix'], $context['subject'], '" />
<input type="hidden" name="icon" value="xx" />
<input type="hidden" name="from_qr" value="1" />
<input type="hidden" name="notify" value="', $context['is_marked_notify'] || !empty($options['auto_notify']) ? '1' : '0', '" />
<input type="hidden" name="not_approved" value="', !$context['can_reply_approved'], '" />
<input type="hidden" name="goback" value="', empty($options['return_to_post']) ? '0' : '1', '" />
<input type="hidden" name="num_replies" value="', $context['num_replies'], '" />';
// Guests just need more.
if ($context['user']['is_guest'])
echo '
<strong>', $txt['name'], ':</strong> <input type="text" name="guestname" value="', $context['name'], '" size="25" />
<strong>', $txt['email'], ':</strong> <input type="text" name="email" value="', $context['email'], '" size="25" />';
echo '
<div class="floatleft" style="border: none; width: 60%;">
<textarea cols="75" rows="1" style="width: 95%; height: 15px;" name="message" tabindex="1"></textarea>
</div>
<div class="floatleft" style="border: none; width: 20%;">
<input type="submit" name="post" value="', $txt['post'], '" onclick="return submitThisOnce(this);" accesskey="s" tabindex="2" />
<input type="submit" name="preview" value="', $txt['preview'], '" onclick="return submitThisOnce(this);" accesskey="p" tabindex="4" />
</div>';
echo '
<input type="hidden" name="', $context['session_var'], '" value="', $context['session_id'], '" />
<input type="hidden" name="seqnum" value="', $context['form_sequence_number'], '" />
</form>
</div>
</div>';