Você pode usar e adaptar o código abaixo de forma a gerar uma interface personalizada para a página de login do webmail de sua revenda:

[php-tag]
#Login Configuration

#cPanel
$protocol = 'http';
$port = 2082;

#Secure cPanel
#$protocol = 'https';
#$port = 2083;

#WHM
#$protocol = 'http';
#$port = 2086;

#Secure WHM
#$protocol = 'https';
#$port = 2087;

#Webmail
#$protocol = 'http';
#$port = 2095;

#Secure Webmail
#$protocol = 'https';
#$port = 2096;
[php-tag]

[html-tag]
<h1>[Web Hosting Company Name Here]</h1>

<b>cPanel Login</b><br>

[php-tag]
if ($_GET['failed'] == "1") {
?>
<font color=#FF0000>Your login attempt failed!</font>
<?php
}
print "<form action=\"" . $protocol . "://" . $_SERVER['HTTP_HOST'] . ":" . $port . "/login/\"
method=POST>";
[php-tag]
User: <input type=text name=user><br>
Pass: <input type=password name=pass><br>
<?php
print "<input type=hidden name=failurl value=\"http://" .  $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'] . "?failed=1\">";
?>
<input type=submit value=Login>
</form><br>
<a href="cpanel-login.phps">Source</a><br>
Note: This examples requires cPanel 9.4.1 or later!
[html-tag]