StringBuilder sb = new StringBuilder();
        sb.append("{"index" : "indexname", "type": "typename"}");
        sb.append("{"query" : {"match_all" : {}}, "from" : 0, "size" : 10}");
        sb.append("{"index" : "indexname", "type": "typename"}");
        sb.append("{"query" : {"match_all" : {}}}n");

        String query = sb.toString();

        String fullURL = "http://esHost/_msearch";

        log.debug("URL to search: {}.", fullURL);
        log.info(">>>>" + query);

        HttpHeaders headers = new HttpHeaders();
        headers.add("Content-Type", "application/x-ndjson");

        HttpEntity<String> request = new HttpEntity<>(query, headers);

        Map map = restTemplate.postForObject(fullURL, request, Map.class);
...

{"error":{"root_cause":[{"type":"action_request_validation_exception","reason":"Validation Failed: 1: no requests added;"}],"type":"action_request_validation_exception","reason":"Validation Failed: 1: no requests added;"},"status":400}

private static final String NEW_LINE = System.getProperty("line.separator");

        sb.append("{"index" : "indexname", "type": "typename"}").append(NEW_LINE)
        sb.append("{"query" : {"match_all" : {}}, "from" : 0, "size" : 10}").append(NEW_LINE)
        sb.append("{"index" : "indexname", "type": "typename"}").append(NEW_LINE)
        sb.append("{"query" : {"match_all" : {}}}n").append(NEW_LINE);