Check URL and I want to show a messagebox with 403 forbidden

See more:

C#

WebForms

MessageBox

In my web form want to check textBox1 input text for the URL and I want to show a message 403 Forbidden
but I am unsure about what the user needs to type in and what I need to check for in order to display such a message to the user.

What should I do at the space of "?????"

And Is there anything that can show error codes of a URL visited automatically without having to create a message box?

What I have tried:

if (textBox1.Text == ????? )
                {
                    string message2 = "Status Code";
                    string caption2 = "403 Forbidden";
                    MessageBoxButtons buttons2 = MessageBoxButtons.OK;
                    DialogResult result2;
                    result2 = MessageBox.Show(message2, caption2, buttons2);
                    
                }