IDP Initiated Login is a feature that allows you to set up an endpoint on the Keycloak server that will log you into a specific application/client. In the Settings tab for your client, you need to specify the IDP Initiated SSO URL Name. This is a simple string with no whitespace in it. After this you can reference your client at the following URL: root/auth/realms/{realm}/protocol/saml/clients/{url-name}

If your client requires a special relay state, you can also configure this on the Settings tab in the IDP Initiated SSO Relay State field. Alternatively, browsers can specify the relay state in a RelayState query parameter, i.e. root/auth/realms/{realm}/protocol/saml/clients/{url-name}?RelayState=thestate.

When using identity brokering, it is possible to set up an IDP Initiated Login for a client from an external IDP. The actual client is set up for IDP Initiated Login at broker IDP as described above. The external IDP has to set up the client for application IDP Initiated Login that will point to a special URL pointing to the broker and representing IDP Initiated Login endpoint for a selected client at the brokering IDP. This means that in client settings at the external IDP:

Please note that you can import basic client settings from the brokering IDP into client settings of the external IDP - just use SP Descriptor available from the settings of the identity provider in the brokering IDP, and add clients/client-id to the endpoint URL.