Puoi provare con questo codice che aveva postato Skeggia molto tempo fa, e che funzionava allora...:

<!-- Private Area.Login: START -->
<div id="imLogin" style="width: 600px">
<form method="post" action="imlogin.php" onsubmit="_jq(this).attr('action', x5engine.settings.currentPath + 'imlogin.php'); return (x5engine.utils.isOnline() || !x5engine.utils.showOfflineMessage(x5engine.l10n.get('blog_offline_message')));">
<div class="imLoginBlock" style="width: 100%;">
<label for="imUname" style="float: left; vertical-align: top; display: inline-block; font-weight: bold; font-style: normal;font-family:Tahoma; font-size:10pt; color: blue; margin: 3px 3px 2px 0; width: 80px;">Utente:</label>
<input type="text" name="imUname" id="imUname" style="float: left; width: 635px; margin-right: 10px; vertical-align: top; padding-top: 2px; padding-bottom: 2px; width: 150px"> <label for="imPwd" style="float: left; vertical-align: top; display: inline-block; font-weight: bold; font-style: normal;font-family:Tahoma; font-size:10pt; color: blue; margin: 3px 3px 2px 0; width: 80px;">Password:</label>
<input type="password" name="imPwd" id="imPwd" style="float: left; width: 635px; margin-right: 10px; vertical-align: top; padding-top: 2px; padding-bottom: 2px; width: 150px">
<input type="submit" value="Accedi" class="imLoginSubmit" style="font-weight: bold; font-style: normal;border: 1px solid #ccc; background-color: #DDDDDD; font-family:Tahoma; font-size:10pt; color: red">
</div>
</form>
</div>
<!-- Private Area.Login: END -->