How to make your params show up in the Spring security default LoginController's login/auth page

In my app, I have a user submit a form with a text field first. At the point, if they're not registered yet, Spring will render the login/auth page. The problem is, I wanted that input to show up on the login/auth page, and by default that param doesn't show up (because the Spring Security plugin renders the login/auth page).

So what do you do? Would you have to dig deep into the Grails/Spring Security code? You probably could but you could actually be wasting time! If one looks at the docs for the Grails Spring Security Core documentation, you could see that there is a param you can set:

Property	Default Value	Meaning
apf.filterProcessesUrl	'/j_spring_security_check'	Login form post URL, intercepted by Spring Security filter.
apf.usernameParameter	'j_username'	Login form username parameter.
apf.passwordParameter	'j_password'	Login form password parameter.
apf.allowSessionCreation	true	Whether to allow authentication to create an HTTP session.
apf.postOnly	true	Whether to allow only POST login requests.
failureHandler. defaultFailureUrl	'/login/authfail?login_error=1'	Redirect URL for failed logins.
failureHandler. ajaxAuthFailUrl	'/login/authfail?ajax=true'	Redirect URL for failed Ajax logins.
failureHandler. exceptionMappings	none	Map of exception class name (subclass of AuthenticationException) to which the URL will redirect for that exception type after authentication failure.
failureHandler. useForward	false	Whether to render the error page (true) or redirect (false).
successHandler. defaultTargetUrl	'/'	Default post-login URL if there is no saved request that triggered the login.
successHandler. alwaysUseDefault	false	If true, always redirects to the value of successHandler. defaultTargetUrl after successful authentication; otherwise redirects to to originally-requested page.
successHandler. targetUrlParameter	'spring-security-redirect'	Name of optional login form parameter that specifies destination after successful login.
successHandler. useReferer	false	Whether to use the HTTP Referer header to determine post-login destination.
successHandler. ajaxSuccessUrl	'/login/ajaxSuccess'	URL for redirect after successful Ajax login.
auth.loginFormUrl	'/login/auth'	URL of login page.
auth.forceHttps	false	If true, redirects login page requests to HTTPS.
auth.ajaxLoginFormUrl	'/login/authAjax'	URL of Ajax login page.
auth.useForward	false	Whether to render the login page (true) or redirect (false).
logout.afterLogoutUrl	'/'	URL for redirect after logout.
logout.filterProcessesUrl	'/j_spring_security_logout'	Logout URL, intercepted by Spring Security filter.
logout.handlerNames	['rememberMeServices', 'securityContextLogoutHandler']	Logout handler bean names. See Logout Handlers
adh.errorPage	'/login/denied'	Location of the 403 error page.
adh.ajaxErrorPage	'/login/ajaxDenied'	Location of the 403 error page for Ajax requests.
ajaxHeader	'X-Requested-With'	Header name sent by Ajax library, used to detect Ajax.
redirectStrategy. contextRelative	false	If true, the redirect URL will be the value after the request context path. This results in the loss of protocol information (HTTP or HTTPS), so causes problems if a redirect is being performed to change from HTTP to HTTPS or vice versa.
switchUser URLs		See Switch User, under Customizing URLs.

Notice the auth.useForward flag.
All you need to do is to set this in your Config.groovy:

grails.plugins.springsecurity.auth.useForward = true

..and you will see your param(s) in the login/auth page, since it's a redirect and not a full render.