Pass Hidden Field value to C# at page load then use it in page load

at JavaScript

    <script>SetHiddenVariable()</script>
    <script type="text/javascript">
        function SetHiddenVariable() {
            var jsVar = "dotnetcurry.com";

            var hiddenControl = '<%= inpHide.ClientID %>';
        document.getElementById(hiddenControl).value = jsVar;
    }
    </script>

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at html form

 <input id="inpHide" type="hidden" runat="server" />
        <asp:TextBox ID="txtJSValue" runat="server"></asp:TextBox>
        <asp:Button ID="btnJSValue" Text="Click to retreive Javascript Variable" runat="server" onclick="btnJSValue_Click"/>

-----------------------------------------

at c#

   protected void btnJSValue_Click(object sender, EventArgs e)
        {
            txtJSValue.Text = inpHide.Value;
        }