When and how to replace these placeholders in the configuration?

In the README, there is an example configuration:

{
 "api_version": 1,
 "default_tags": [{"name": "geo"}, {"name": "namibia"}],
 "default_groups": ["science", "spend-data"],
 "default_extras": {"encoding":"utf8", "harvest_url": "{harvest_source_url}/dataset/{dataset_id}"},
 "override_extras": true,
 "organizations_filter_include": [],
 "organizations_filter_exclude": ["remote-organization"],
 "user":"harverster-user",
 "api_key":"<REMOTE_API_KEY>",
 "read_only": true,
 "remote_groups": "only_local",
 "remote_orgs": "create"
}

And in the Web URL /harvest:

I don't know when and how to replace the {harvest_source_url} and {dataset_id}.

ckan / ckanext-harvest

When and how to replace these placeholders in the configuration? #531