open a new window using body onload
<p>Edit- here's the code</p>

<pre><code>&lt;html&gt; 
&lt;head&gt; 

&lt;script type="text/javascript"&gt; 
var prodURL = "https://blah"; 

function postwith (to, params) { 
var myForm = document.createElement("form"); 
myForm.method = "post"; 
myForm.target = "_self"; 
myForm.action = to; 

for (var p in params) { 
var myInput = document.createElement("input"); 
myInput.setAttribute("name", p); 
myInput.setAttribute("value", params[p]); 
myInput.setAttribute("type", "hidden"); 
myForm.appendChild(myInput); 
} 

document.body.appendChild(myForm); 
myForm.submit(); 

} 
&lt;/script&gt; 

&lt;style type="text/css"&gt; 
#overlayPageLoad { 
background: none repeat scroll 0 0 white; 
height: 100% !important; 
left: 0; 
position: absolute; 
text-align: center; 
top: 0; 
width: 100% !important; 
} 
#overlayPageLoad .middle { 
color: #888888; 
font-size: 16px; 
left: 0; 
position: absolute; 
top: 50%; 
width: 100%; 
} 
&lt;/style&gt; 

&lt;/head&gt; 

&lt;body onload= "postwith(prodURL, {SF_SESSION:'{!$Api.Session_ID}',SF_ENDPOINT:'{!$Api.Partner_Server_URL_120}'})"; &gt; 

&lt;div id="overlayPageLoad"&gt; 
&lt;span class="middle"&gt; 
&lt;img src="https://prettypicture"&gt; 
&amp;nbsp;Connecting... 
&lt;/span&gt; 
&lt;/div&gt; 

&lt;/body&gt; 
&lt;/html&gt;
</code></pre>
