>>700さんのリンク先参考に見よう見まねでとりあえずやってみたんですけど、いまいちcookie_jarとかよくわからない・・・

require 'mechanize'
require 'yaml'

agent = Mechanize.new
agent.user_agent_alias = "Windows Mozilla"
agent.get('https://mevius.5ch.net/test/read.cgi/tech/1578068134/l50')
cookie_str = agent.cookies.map{|e| e.set_cookie_value}.join(", ")
#puts cookie_str

agent2 = Mechanize.new
HTTP::Cookie.parse(cookie_str, "https://mevius.5ch.net/test/read.cgi/tech/1578068134/l50";).each{ |cookie| agent2.cookie_jar << cookie }

cookie_yml = agent.cookies.map{ |e| e.to_yaml }.join
#puts cookie_yml

agent2.cookie_jar << YAML.load(cookie_yml)

#pp agent2.cookie_jar

agent2.user_agent_alias = "Windows Mozilla"
page = agent2.get('https://mevius.5ch.net/test/read.cgi/tech/1578068134/l50')


form = page.form_with(:action=>'//mevius.5ch.net/test/bbs.cgi')
form.field_with(:name=>'MESSAGE').value = 'テスト'
next_page = agent2.submit(form)
form2 = next_page.form_with(:action=>'../test/bbs.cgi?guid=ON')