$ curl -X GET "localhost:9200/_xpack/license/start_trial" -vv
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 9200 (#0)
> GET /_xpack/license/start_trial HTTP/1.1
> Host: localhost:9200
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 405 Method Not Allowed
< Allow: POST
< content-type: application/json; charset=UTF-8
< content-length: 118
<
* Connection #0 to host localhost left intact
{"error":"Incorrect HTTP method for uri [/_xpack/license/start_trial] and method [GET], allowed: [POST]","status":405}

$ bin/elasticsearch --version
Java HotSpot(TM) 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
Version: 6.3.0, Build: default/tar/424e937/2018-06-11T23:38:03.357887Z, JVM: 10
$ java -version
java version "10" 2018-03-20
Java(TM) SE Runtime Environment 18.3 (build 10+46)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)
$ uname -a
Darwin Davids-MacBook-Pro-3.local 17.5.0 Darwin Kernel Version 17.5.0: Fri Apr 13 19:32:32 PDT 2018; root:xnu-4570.51.2~1/RELEASE_X86_64 x86_64
$ curl http://localhost:9200/_cat/plugins
$