[keycloak-user] identity broker SAML logout with redirect

[Madhu]

Hi
I am looking at configuring keycloak as an identity broker usign SAML, the external idm i am integratin supports a logout url and also takes a queryParam for redirecting the user after logut.
how can i pass the redirectURL of keyclock's logout to this external IDM in the broker configuration.
i tried  parameterizing the  logouturl link in my keycloak config by specifying ${redirectURL} and this did not work out of the box, is there any way i can pass the redirect url of keycloak to external idm as a query para? e.g  https://externalIDM/logout?logoutURk=${redirectURL} 
My external idm does not seem to support back channel logouts.
IF i donot configure logut, the external idm does not logout the user, if id do not specify the postlogout url to external idm ,  the user is redirected to the external idms default login page. my intention is to redirect the user to keycloak login page with the app/client context..
Regards,Madhu