Its not that simple as calling webservice methods from Input button or server-side button control click event.

Here I found pageLoad() (case sensitive) event  which is specifically accomodated with Atlas to support the javascript "onLoad" events e.g. calling a WebMethod from onLoad event of body element.

Only "pageLoad()" can call the WebMethod in the following example:-

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
<title>Untitled Page</title>
<atlas:ScriptManager ID="scriptManager" runat="server" EnableScriptComponents="false" >
<Services>
<atlas:ServiceReference Path="SimpleService.asmx" Type="text/javascript" />
</Services>
</atlas:ScriptManager>
<script type="text/javascript" language="JavaScript">
//if we call this on body onLoad will raise error
function OnStart() {
//Call script proxy passing the input element data
requestSimpleService = Quickstart.Samples.SimpleService.EchoString("Hello",OnComplete);
return false;
}

//only pageLoad will call the webmethod
function pageLoad()
{
//Call script proxy passing the input element data
requestSimpleService = Quickstart.Samples.SimpleService.EchoString("pageLoad event is supported by Atlas!!",OnComplete);
return false;
}

function OnComplete(result) {
alert(result);
}
</script>
</head>
<body onload="pageLoad()">

<form id="form1" runat="server">
<div>
</div>
</form>
</body>
</html>