This is technically easy for someone with some Ruby programming experience. Just write a plugin, responding to the :post_created event, write some code like this:
DiscourseEvent.on(:post_created) do |*params|
post, opt, user = params
if post.raw.include "foo"
PostCreator.create!(
User.find_by(id: bot_id),
topic_id: post.topic_id,
raw: "bar",
)
end
end