CommandArgument property for button is used to pass EventArgs, though its a ServerSide property and cannot be accessed through Javascript.
I believe, best way would be to use a hiddenfield and set value through JavaScript. Here's the sample code...
<script type="text/javascript">
function SetHiddenField()
{
document.getElementById('hfSample').value = "value to assign";
document.getElementById('form1').submit();
}
</script>
<body onload="SetHiddenField();">
<form id="form1" runat="server">
<div>
<input id="hfSample" type="hidden" value="" />
<input id="btnSubmit" type="button" onclick="SetHiddenField();" />
</div>
</form>
</body>