>> Net::HTTP.instance_methods.grep(/post/)
=> ["post", "post2", "request_post", "enable_post_connection_check", "enable_post_connection_check="]
>> Net::HTTP.methods.grep(/post/)
=> ["post_form"]
Which means that post_form is not an instance method of Net::HTTP but a class method. However, there is an instace method Net::HTTP#post, which may be what you're looking for.
Regards,
Michael Guterl