How to call _secureURL

I am trying to do redirects to the calling page inside my Coldbox
admin page. If foo.com/test before logging in they will they will not
always be sent to foo.com/index always like they are now. I am using
Coldbox's built in security interceptor. So my question is what is the
syntax to call _secureURL inside of the form using a hidden form
field?
Thanks for your help.

I did a dump of the request collection scope inside the login form and
I see the _secureURL key
when I try and access something besides the index page. I then did a
dump of the session scope and I see it is set to autopurge.
So, it is on being available on the login form page. How can I
override this autopurge setting?
Thanks!

With Luis Majano's help I have been able to solve this.

The first thing I did was to verify that _secureURL was being created
on the secure login form by calling something other than the default
setNextEvent in my handler.
I did this by just pasting the link into the browser window and
dumping the <cfdump var="#rc#"> request collection in the login form.
I could see the link coming back in the _sercureURL key of the request
collection.
I then just needed to persist that key to the handler that gets called
when I click the login submit button. The code I used to persist the
_secureURL key is <cfset flash.keep("_securedURL") />

In my login handler I added this code.
Check to make sure _securedURL is not an empty string
      <cfif Len(flash.get("_securedURL",""))>
      <cfset
setNextEvent(event=ReplaceNoCase(flash.get("_securedURL",""), "/
index.cfm/", "")) />
      <cfelse>
<cfset setNextEvent("admin.index") />
</cfif>
I am replacing the '/index.cfm/' because I am using ISAPPI rewrite to
remove the index.cfm.
Thanks, Luis for pointing me in the right direction