External IDP(Siteminder) initiated SSO with Keycloak application

I have an application configured in Keycloak with SAML protocol. I am trying to achieve External IDP (siteminder) initiated SSO for the application. At siteminder, configured the ACS url in the format broker-root/auth/realms/smp/broker/idp_homeview/endpoint/clients/{client-id} as mentioned in the docs at https://github.com/keycloak/keycloak-documentation/blob/master/server_admin/topics/clients/saml/idp-initiated-login.adoc.

The externalIDP is authenticating the users and posting the SAML assertion response to the above url and I am seeing 415 Unsupported Media type. Do I need to update any configuration of the application under clients in Keycloak to avoid this?