Hi, you can do an "on after change" event on you text box, build your URL in xhtml(<a href:"http:myURL"> Click here</a> and modify the innerXML on a Rich text field (if you use C# 2007), and make some conditionnal formatting on both. For your text field, you hide it when it's not blank, and your RT field, your hide it if it's blank.
An easiest way is to add a button and launch an IE process of your URL: Process.Start("IExplore.exe", MyUrl);