use JavaScript for this
<script type="text/javascript">
function OpenPage() {
this.parent.document.getElementById("frm2").src = "PostRequirement.aspx";
}
</script>
and call this on Clientclick of button like this
<asp:button text="Click" runat="server" id="cmd1" onclientclick="javascript:OpenPage();"/>
Hope this will work for you otherwise let me know the issue.
--Pankaj