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agent#getの引数で、form_withの引数にname属性を設定する

require "mechanize"
agent = Mechanize.new
agent.get("https://www.hoge.com/admin/login") do |page|

    login_result = page.form_with(name:"loginform") do |login|
        login.field_with(name: "user[email]").value = user
        login.field_with(name: "user[password]").value = pass
    end.submit
end

参考