Try like this
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script src="jquery-1.10.2.js" type="text/javascript"></script>
<script language="Javascript">
function country() {
var country = "HI";
var hiddenControl = '<%= inpHide.ClientID %>';
document.getElementById(hiddenControl).value = country;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:HiddenField ID="inpHide" runat="server" />
<asp:Label ID="lblMsg" runat="server" />
<asp:Button ID="btn" Text="some" runat="server" OnClientClick="country();"
onclick="btn_Click" />
</form>
</body>
</html>
protected void btn_Click(object sender, EventArgs e)
{
lblMsg.Text = inpHide.Value;
}