[keycloak-user] Setting custom redirect URI in SAML Identity Provider

Hi, We use a SAML Identity Provider configuration in Keycloak to broker identities to an external SAML-based Identity Provider. This works fine but now we have the requirement that after authentication the user needs to be redirected first to a reverse-proxy and only then back to us (as in: Keycloak). I.e. we need to configure a custom redirect URI in our SAML Identity Provider in Keycloak.. However this redirect URI seems to be generated on-the-fly in Keycloak and the hostname part seems always set to the host where Keycloak runs on? Our question is: is this redirect URI configurable at all and if not, how could we go about setting it ourselves (the hostname part at least)? I guess that we would need to create our own custom Identity Provider (e.g. extension of the SAMLIdentityProvider and related Java classes) and install this in Keycloak?