Use of eventId in unsolicited for MFA flow

[Martin Leonhartsberger]

Hello,

I have successfully implemented an mfa flow for a UI selection of the login flow.
A button on the login.vm in the Password flow signals back to the MFA Flow to select a different flow.
Basically, this is implemented as the sample in documentation, and it works fine.

/authn/mfa-authn-config.xml
<util:map id="shibboleth.authn.MFA.TransitionMap">

       <entry key="">
<bean parent="shibboleth.authn.MFA.Transition" p:nextFlow="authn/Password" />
      </entry>

<entry key="authn/Password">
    <bean parent="shibboleth.authn.MFA.Transition">
        <property name="nextFlowStrategyMap">
            <map>
                <entry key="methodSAML" value="authn/SAML" />
            </map>
        </property>
    </bean>
</entry>

login.vm
<form action="$flowExecutionUrl" method="post">
     <button class="form-element form-button" type="submit" name="_eventId_methodSAML">
            Use IDP XY
      </button>
  </form>

Is there a possibility, to encode this signal into an unsolicited URL to skip the UI selection step?
e.g. https://idp.localhost/idp/profile/SAML2/Unsolicited/SSO?providerId=<sp-entity-id>&_eventId_methodSAML<https://idp.localhost/idp/profile/SAML2/Unsolicited/SSO?providerId=%3csp-entity-id%3e&_eventId_methodSAML>

I have tried following, though this does not honor the eventid parameter (probably needs to be POST and already in a flow?).
Flow selection via relying party settings for the SP is not an option, I would need this selectable via UI for the user and via URL parameter.

<util:map id="shibboleth.authn.MFA.TransitionMap">
        <entry key="">
        <bean parent="shibboleth.authn.MFA.Transition">
                <property name="nextFlowStrategyMap">
                    <map>
                        <!-- Maps event to a flow -->
                        <entry key="proceed" value="authn/Password" />
                        <entry key="methodSAML" value="authn/SAML" />
                    </map>
                </property>
            </bean>
        </entry>

Thanks for any hint!

Best regards,
Martin
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