how do you mean by a custom url are you referring to say a textbox where the user types in the url they wish to navigate to if so then you do just that my friend, place a textbox on the form, perhaps also place a button on the form to "go" to that url. If you are using the WebBrowser control then simply, on the button click event, tell the webbrowser to navigate to that url....
private void button1_click(object sender, EventArgs e)
if (this.theTextBox.Text.Length > 0)
you may wish to do some url validation before hand however this was a short/simple example. Is this the thing you are after