How to configure a backchannel URL for a specific client and what is the correct value for the Spring Boot/Spring Security Plugin

I want to logout a Spring Boot Server that uses the Spring Boot / Spring Security Keycloak Plugin when a user logs out from Keycloak session.

I found out that the OIDC specification provides a backchannel to logout specific clients. In version 12 there was a specific setting for the backchannel logout URL in the client configuration. In version 16 this setting is gone.
Keycloak 12:
Keycloak 16:

Therefore my first question is: Where to configure this setting?
And the second question: What is the URL for java server with Spring Boot/Spring Security Keycloak Plugin? So http://:/<this part is what I don’t know>
I couldn’t find anything related to this in the Keycloak documents.