How to set Port Number when using FtpWebRequest for uploading files to FTP in ASP.Net

Thanks for this:

http://www.aspsnippets.com/Articles/Uploading-Files-to-FTP-Server-programmatically-in-ASPNet-using-C-and-VBNet.aspx

Unfortunately I get an error while uploading:

Invalid URI: Invalid port specified.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.UriFormatException: Invalid URI: Invalid port specified.

FtpWebRequest request = (FtpWebRequest)WebRequest.Create(ftp + ftpFolder + fileName);

My FTP-address is:
ftp://ftp.mydomain.dk:4001