Show red border on textbox on error - asp.net

Question

I am trying to show an error by changing the color of the border of a text box. I have found a lot of posts on how to do it, but I have never done anything with 'Validators' so I am confused where to start, or where to put the code.
Similar posts:
Post One
Post Two
My main question(s) are, where do I put the JS code? And secondly, how do I call the code?
Current Code
If TicketNumber.Text = String.Empty = True Then
NotificationLabel.Text = "Ticket Number must be filled out"
Exit Sub
End If

Answer 1

A RequiredFieldValidator could do it with some tricks explained here, and here. Another way is to use a CustomValidator like the following.
aspx mark-up
<asp:TextBox runat="server" ID="tbText" />
<asp:CustomValidator ID="cvCustom" ErrorMessage="Ticket Number must be filled out"
ControlToValidate="tbText" runat="server" OnServerValidate="ValidateInput"
ClientValidationFunction="ValidateInput" ValidateEmptyText="True" />
<asp:Button Text="My Button" runat="server" />
Javascript
<script type="text/javascript">
function ValidateInput(sender, args) {
if (args.Value == '') {
args.IsValid = false;
document.getElementById('<%= tbText.ClientID %>').style.borderColor = "red";
return false;
}
return true;
}
</script>
Code Behind (C#)
protected void ValidateInput(object source, ServerValidateEventArgs args)
{
if(string.IsNullOrWhiteSpace(args.Value))
{
args.IsValid = false;
tbText.Style["border-color"] = "red";
}
else
{
args.IsValid = true;
}
}