Sadly, yes. Each user would have to sign up as a Amazon Developer, sign up for the Lambda site (which requires credit card information) and follow a ridiculously long set of instructions to get the code running. OpenSprinkler would have to be mapped to an external IP with Duckdns or something similar. The external address and port would then be mapped to the skill along with the MD5 password. Because of the required mapping, this would never be a skill approvable by Amazon for publication.
Its a lot of work for most users to shut off or turn on the sprinklers orally! If OpenGaragedoor uses JSON strings, that might make more sense. You could open the garage door with your hands full.
Ray, here is a link for setting up a skill for a Kodi media player. I am including it because the instructions are great. Very detailed. The steps apply to any unpublished skill. https://www.howtogeek.com/249336/how-to-control-your-kodi-media-center-with-an-amazon-echo/
All that being said, It would be fairly complex for you guys to incorporate this. There would have to be some sort of phone based “handshake” like NEST uses to get the IP address and MD5 password into the Skill.