Web form autofill - live.com

I develop web browser plugin which can autofill html web forms(username/password). The plugin works good except several sites. For example if I try login.live.com then correct username appears but I see "Please enter a valid email address, phone or Skype name" error after click "Next" button. If I manually enter any character and delete it then I don't see the error. How can I bypass the javascript input detection mechanism?