curl -X POST --url http://127.0.0.1:8001/services/<a service>/plugins
  --form "name=ssl-api"
  --form "config.cert=@/path-to-key-file.key"
  --form "config.key=@/path-to-crt-file.crt"

{
  "config":"plugin 'ssl-api' not enabled; add it to the 'plugins' configuration
  property"
}