CustomAuthenticator doesn't work for URL /Thingworx/FormLogin

Hi, 

I created a CustomAuthenticator and want to catch the login request to redirect to another page.  however, I found the request /Thingworx/FormLogin won't be intercepted by matchesAuthRequest

matchesAuthRequest(HttpServletRequest httpRequest)

that's strange,  the other GET request , for example both of /Thingworx/Home and /Thingworx/Composer can be intercepted by this method .  

Is the URL /Thingworx/FormLogin ignored by the servlet filter intently ? 

Regards,

Sean