The Open Redirect warning is issued in the following situations:

Do one of the following: replace the 'External Site' element with a well-known page in your application; replace the 'External Site' element with another 'External Site' element that does not receive a URL as an input and that redirects to a static URL instead; or use the ReplaceURLDomain function from the HTTPRequestHandler extension module to enclose the value of the parameter.

See Also

ReplaceURLDomain function | About Warning Messages