I use a curl command like the following to load (import) content (dashboards, visualization, etc.) into Kibana:
curl -XPOST "http://myserver:5600/api/saved_objects/_import?overwrite=true" -H "kbn-xsrf: true" -H "securitytenant: global" --form file=@mycontent.ndjson
Obviously, you would need to change the host, port and name of the ndjson file…and, potentially, the securitytenant if you want to import to a different tenant.
This API is documented in the Kibana API doc rather than in the Elasticsearch API doc.