I'm not sure what your post_message command is, but if it's something you found in Cinch, don't use it. Instead, use the built-in commands to respond to messages
def on_join(m)
# Don't greet the bot.
unless m.user == @bot
# Reply to the user who joined, the true prepends the line with the user's name
m.reply "Use this link to get more info - http://whatever", true
end
And then:
15:30 -!- Irssi: #bottest: Total of 2 nicks [0 ops, 0 halfops, 0 voices, 2 normal]
15:30 -!- Channel #bottest created Mon Feb 24 15:30:17 2014
15:30 < bot> user: Use this link to get more info - http://whatever