How do I implement Javascript on a textbox inside a gridview?

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C#

ASP.NET

I use this javascript as a field validator

 <script type="text/javascript">
      //Function to allow only numbers to textbox
      function validate(key) {
          //getting key code of pressed key
          var keycode = (key.which) ? key.which : key.keyCode;
          var phn = document.getElementById('txtPhn');
          //comparing pressed keycodes
          if (!(keycode == 8 || keycode == 46) && (keycode < 48 || keycode > 57)) {
              return false;
          }
          else {
              //Condition to check textbox contains ten numbers or not
              if (phn.value.length < 10) {
                  return true;
              }
              else {
                  return false;
              }
          }
      }
</script>

and add the function to the textbox

<asp:TextBox ID="txtTotal" runat="server" Width="131px" BorderStyle="None" Height="18px" onkeypress="return validate(event)"></asp:TextBox>

However, when the control is inside a gridview,

for example the textbox is inside the gridview, how do I call the onkeypress event?