An example on how to send a push notification via One Signal with Nakama server.

onesignal.lua

local nk = require("nakama")

--[[
  Send a push notification to a segment of users via One Signal.
]]--
local function send_push_notification(context, payload)
  local auth = "YOUR ONE SIGNAL REST API KEY"

  local url = "https://onesignal.com/api/v1/notifications"
  local method = "POST"
  local headers = {
    ["Authorization"] = ("Basic %s"):format(auth)
  }
  local content = nk.json_encode({
    app_id = "YOUR ONE SIGNAL APP ID",
    contents = {
      en = "English Message"
    },
    included_segments = {"All"}
  })

  local success, code, headers, resp_body = pcall(nk.http_request, url, method, headers, content)
  if (not success) then
    nk.logger_error(("Failed to send push messages: %q"):format(code))
  elseif (code >= 400) then
    nk.logger_error(("Failed to send push message: %q %q"):format(code, resp_body))
  else
    nk.logger_info(("Success %q %q"):format(code, resp_body))
  end
end
nk.register_rpc(send_push_notification, "one_signal")

For Nakama server 1.0-rc.1 use:

local nk = require("nakama")
local nx = require("nakamax")

--[[
  Send a push notification to a segment of users via One Signal.
]]--
local function send_push_notification(context, payload)
  local auth = "YOUR ONE SIGNAL REST API KEY"

  local url = "https://onesignal.com/api/v1/notifications"
  local method = "POST"
  local headers = {
    ["Authorization"] = ("Basic %s"):format(auth)
  }
  local content = nx.json_encode({
    app_id = "YOUR ONE SIGNAL APP ID",
    contents = {
      en = "English Message"
    },
    included_segments = {"All"}
  })

  local success, err = pcall(nx.http_request, url, method, headers, content)
  if (not success) then
    nk.logger_error(("Failed to send push messages: %q"):format(err))
  end
end
nk.register_rpc(send_push_notification, "one_signal")

The main difference is you'll need to use the nakamax module which includes http_request and json_encode. In the 1.0 release it's been moved into the main nakama module.