Can I generate & transmit ACARS message via software?
You are unlikely to have a permission for sending technical messages over radio channels used by real aircraft.
But if you just want to make your drone to send you ACARS messages over WiFi, there are 144 open source projects currently on GitHub to deal with these messages. Most of them are parsers, not senders, but anyway selecting the best projects and then understanding the code should give enough insights about the message format. Also, parsers can be used for checking that your sender works properly.