repmgr witness register adds a witness server's node record to the repmgr metadata, and if necessary initialises the witness node by installing the repmgr extension and copying the repmgr metadata to the witness server. This command needs to be executed to enable use of the witness server with repmgrd.
When executing repmgr witness register, database connection information for the cluster primary server must also be provided.
In most cases it's only necessary to provide the primary's hostname with the -h/--host option; repmgr will automatically use the user and dbname values defined in the conninfo string defined in the witness node's repmgr.conf, unless these are explicitly provided as command line options.
Execute with the --dry-run option to check what would happen without actually registering the witness server.
$ repmgr -f /etc/repmgr.conf witness register -h node1
INFO: connecting to witness node "node3" (ID: 3)
INFO: connecting to primary node
NOTICE: attempting to install extension "repmgr"
NOTICE: "repmgr" extension successfully installed
INFO: witness registration complete
NOTICE: witness node "node3" (ID: 3) successfully registered