Triggering another authentication flow from authn/Password

[Philip Brusten]

Hi

I want users to be able to trigger an alternative authentication method 
by clicking on a button from the login-view of the authn/Password method.

I configured it based on the MFA-example: Full Control Over Transitions [1]

<entry key="authn/Password">
   <bean parent="shibboleth.authn.MFA.Transition">
     <property name="nextFlowStrategyMap">
         <map>
             <!-- Maps event to a flow -->
             <entry key="ChooseMethod" value="authn/custom" />

             <!-- Maps event to a scripted function bean reference-->
               <entry key="proceed" value-ref="checkSecondFactor" />
         </map>
     </property>
   </bean>
</entry>

In my login.vm view, I added a form like:

<form action="$flowExecutionUrl" method="post">
     <button class="form-element form-button" type="submit" 
name="_eventId_ChooseMethod">
            Use this method
      </button>
  </form>

If I submit the form, I get this error:

ERROR [net.shibboleth.idp.authn:-2] - Uncaught runtime exception - 
org.springframework.webflow.engine.NoMatchingTransitionException: No 
transition found on occurence of event 'ChooseMethod' in state 
'DisplayUsernamePasswordPage' of flow 'authn/Password' -- valid 
transitional criteria are array<TransitionCriteria>[proceed, 
currentEvent.id.startsWith('authn/')] -- likely programmer error, check 
the set of TransitionCriteria for this state
         at 
org.springframework.webflow.engine.TransitionableState.getRequiredTransition(TransitionableState.java:90)

How can I let the MFA TransitionMap handle this event instead of the 
authn/Password flow?

Thx,
Philip

[1] 
https://wiki.shibboleth.net/confluence/display/IDP30/MultiFactorAuthnConfiguration#MultiFactorAuthnConfiguration-FullControlOverTransitions