Confirm Message in web asp.net c#

Question

i have written below code to confirm message before save record into database. but when i impliment inside my project its not working. 

 

Javascript code written into my web page after after head tag.  

i have used (!ispostback) in side my project kindly help me what is wrong. the page is not do any process after button press.  

<script type="text/javascript">
          function ConfirmMessage() {
              var selectedvalue = confirm("Do you want to save data?");
              if (selectedvalue) {
                  document.getElementById('<%=txtconformmessageValue.ClientID %>').value = "Yes";
              } else {
                  document.getElementById('<%=txtconformmessageValue.ClientID %>').value = "No";
              }
          }
    </script>

 added hidden field for the store value. 

<asp:HiddenField ID="txtconformmessageValue" runat="server" />

code behind code for save button code. 

if (txtconformmessageValue.Value == "Yes")
                   {
                       Page.ClientScript.RegisterStartupScript(this.GetType(), "Confirm value", "alert('Record Has Been Saved')", true);
                   }
                   else
                   {
                       return;
                       //Page.ClientScript.RegisterStartupScript(this.GetType(), "Confirm value", "alert('You have clicked on CANCEL.')", true);
                   }