Below you can see how this would work, followed by some sample source code.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head><title> Untitled Page </title> <style type="text/css"> .style1 { width: 97px; } em { color:Red; font-style:italic; font-weight:bold; } </style> <script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script> <script type="text/javascript" language="javascript"> function SaveAndClose() { var newValue = new Array(); var Host = window.location.hostname; Host = Host.toLowerCase(); if (Host.indexOf("dev") !== -1) { Host = "http://www.mydevsite.com"; } else if (Host.indexOf("qa") !== -1) { Host = "http://www.myqasite.com"; } else { Host = "http://www.mylivesite.com"; } if ($("#User").val().length > 0) { newValue[0] = Host + "/feed.ashx?type=" + $("#Type").val() + "&user=" + $("#User").val(); window.returnValue = newValue; self.close(); } else { alert("You need to fill in the user field"); } } function HideUserBox() { $("#User").hide(); $("#ContextUser").html("Username is not required for this type of feed"); $("#User").val(""); } function ShowUserBox() { $("#User").show(); $("#ContextUser").html(""); } function getParameterByName(name) { name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]"); var regexS = "[\\?&]" + name + "=([^&#]*)"; var regex = new RegExp(regexS); var results = regex.exec(window.dialogArguments.customArguments.fieldValue); if (results == null) return ""; else return decodeURIComponent(results[1].replace(/\+/g, " ")); } function SetContextHelp() { var str = "<h1>Help</h1>Select the type of feed you would like to use."; var Type = $("#Type").val(); switch (Type) { case "Facebook": str = "<h1>FaceBook<\/h1>All you need to enter for the User field is the numeric part so for the test feed <b>http:\/\/www.dummyfacebookfeedsite.com\/feeds\/feed<em>24<\/em>.xml<\/b> it would be <em>24<\/em><\/p>"; ShowUserBox(); break; case "Investis": str = "<h1>Investis<\/h1>No additional parameters are required<\/p>"; HideUserBox(); break; case "Taleo": str = "<h1>Taleo<\/h1>No additional parameters are required<\/p>"; HideUserBox(); break; case "Twitter": str = "<h1>Twitter<\/h1>All you need to enter for the User field is the part after twitter.com\/ so for the test feed <b>https:\/\/twitter.com\/<em>MyUsername<\/em><\/b> it would be <em>MyUsername<\/em><\/p>"; ShowUserBox(); break; case "Youtube": str = "<h1>Youtube<\/h1>All you need to enter for the User field is the part after \/user\/ so for the test feed <b>http:\/\/www.youtube.com\/user\/<em>MyUsername<\/em><\/b> it would be <em>MyUsername<\/em><\/p>"; ShowUserBox(); break; } $("#ContextHelp").html(str); } $(document).ready(function () { $("#User").val(getParameterByName("user")); $("#Type").val(getParameterByName("type")); SetContextHelp(); }); </script> </head> <body style ="padding:8px;"> <form name="form1" method="post" action="default.aspx" id="form1"> <div> <input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTMxODExNTY1MA9kFgICAw9kFgICAQ8QD2QWAh4Ib25jaGFuZ2UFEVNldENvbnRleHRIZWxwKCk7ZGRkZIJa/6lFwnHB6cSID/LnPwxZzMB7" /> </div> <div> <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWBwL729DXAwLYobCbAwKbwILMBwL7reHFDQKNhbybDAKEnqumCgL8gpeMB6O7pPSCvvDDegx8LyqfWc6lzux3" /> </div> <div id="Main"> <table border="0" width = "100%"> <tr> <td>Type</td> <td> <select name="Type" id="Type" onchange="SetContextHelp();"> <option value="Facebook">Facebook</option> <option value="Investis">Investis</option> <option value="Taleo">Taleo</option> <option value="Twitter">Twitter</option> <option value="Youtube">Youtube</option> </select> </td> </tr> <tr> <td>User</td> <td> <input name="User" type="text" id="User" /><p id = "ContextUser"></p> </td> </tr> <tr> <td class="style1"> </td> <td> <input type="button" value="Save" onclick="SaveAndClose()" /> </td> </tr> </table> <p style ="padding:8px;background-color:#dbdbdb;" id="ContextHelp"></p> </div> </form> </body> </html>