This is an example of using Mechanize 2.0 and posting a form.

gistfile1.rb

require 'rubygems'
require 'mechanize'
require 'net/https'

agent = Mechanize.new
agent.user_agent_alias = 'Mac Safari'
agent.read_timeout = 60
agent.follow_meta_refresh=true
page = agent.get("https://the.website.com/login.do")
current_form=page.forms.first
current_form.username = "myusername"
current_form.password = "mypassword"

newpage = agent.submit current_form

newreq = Mechanize.new
newreq.cookie_jar.add(URI.parse("https://the.website.com/"),agent.cookies[0])
newreq.cookie_jar.add(URI.parse("https://the.website.com/"),agent.cookies[1])
hash = {
  "individualIds" => "",
  "groupIds" => "",
  "publicGroupIds" => "",
  "aliasIds" => "",
  "phoneNumbers" => "9074339549",
  "foreignGroupNames" => "",
  "message.parentMsgId" => "",
  "to" => "",
  "message.subject" => "test subject",
  "message.text" => "test message"
}

headers = {"X-Requested-With" => "XMLHttpRequest", "Content-Type" => "application/x-www-form-urlencoded; charset=UTF-8"}

lastpage = newreq.post("https://the.website.com/pri/messages/do-compose.do?format=json", hash ,headers)