Question seems somewhat unclear,but i think you need to prompt confirmation to user for the input and store if he agrees or not inside hidden field. If so,here is the modified code. Let me know if you still need some help here.
<script>
function myFunction() {
var Txt = document.getElementById("textbox").value;
var r=confirm("Numbers: " + Txt);
document.getElementById('<%=hdnsubmit.ClientID%>').value=r;
}
</script>
Regards..