In the disconnected section of the agent-based installer documentation there is a description of using oc-mirror and how to get the ICSP or IDMS from the output of the command:
https://docs.redhat.com/en/documentation/openshift_container_platform/4.18/html/installing_an_on-premise_cluster_with_the_agent-based_installer/understanding-disconnected-installation-mirroring#configuring-the-agent-based-installer-to-use-mirrored-images

This is valid for oc-mirror --v1. In 4.18 oc-mirror -v2 was released and the mechanism is different. Both methods should be described. For oc-mirror --v2 the file in

working-dir/cluster-resources/idms-oc-mirror.yaml

should be used. The section including "imageDigestMirrors:" should be used in install-config.yaml.

For reference there is also an example of that file here
https://developers.redhat.com/articles/2024/10/14/how-oc-mirror-version-2-enables-disconnected-installations-openshift-416#determine_what_you_want_to_mirror_via_imageset_config