Here is the link to the thread where I found the script.
http://www.directadmin.com/forum/showthread.php?s=&threadid=8305
Here is the code posted by Eris:
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<html>
<head>
<title>DirectAdmin Login</title>
<style>
*{ FONT-SIZE: 8pt; FONT-FAMILY: verdana; }
b { FONT-WEIGHT: bold; }
.listtitle { BACKGROUND: #425984;
COLOR: #EEEEEE;
white-space: nowrap; } td.list { BACKGROUND: #EEEEEE; white-space: nowrap; } </style>
</head>
<body onLoad="document.form.username.focus();">
<script type='text/javascript'>
function changeAction()
{
var x=document.forms.form
var t=x.domain.value
//if you are using SSL its
//hasn't been tested (My provider is running under
http://
https://www."+ t + ":2222/CMD_LOGIN"
var p="http://www."+ t + ":2222/CMD_LOGIN"
x.action=p
}
</script>
<center><br><br><br><br>
<h1>DirectAdmin Login Page</h1>
<table cellspacing=1 cellpadding=5>
<tr>
<td class=listtitle colspan=2>
Please enter your Username and Password</td>
</tr>
<form action="" method="POST" name="form" onsubmit="changeAction()">
<input type=hidden name=referer value="/">
<tr><td class=list align=right>Username:</td>
<td class=list><input type=text name=username></td></tr>
<tr><td class=list align=right>Password:</td>
<td class=list><input type=password name=password></td></tr>
<tr><td class=list align=right>Domain:</td>
<td class=list><input type=text name=domain></td></tr>
<tr><td class=listtitle align=right colspan=2><input type=submit value='Login'></td></tr>
</form>
</table>
</center>
</body>
</html>
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