There's a bug in the URL validation code.
When the code was written, it checks for basic URL syntax before redirecting, ensuring the URL starts with http://. If your URL doesn't start with that, you have two options:
Make sure your redirect URL starts with http://
Open FormMail and find this line (line 204 in standard distribution):
if ($CONFIG{'redirect'} =~ /http\:\/\/.*\..*/) {
Change it to:
if ($CONFIG{'redirect'}) {
This removes the URL validation and allows any URL or URI in the redirect form field.