1. Add the using reference to the top of your pages .cs file
using System.Web.UI.HtmlControls;
2. In your Page_Load function of the .cs file add
((HtmlGenericControl) this.Page.Master.FindControl("PageBody")).Attributes.Add("onload", "load()");
3. Make sure your master page BODY has the the matching id from above. in this case: ID="PageBody" runat="server"
4. In your aspx file, add you load function. something like
<script type="text/javascript">
function load() {
alert('page loaded');
}
</script>