A few lines about what you are building, what stage it is in, and what you expect from the collaboration is usually the best starting point.
Contact
Say hello.
Email is the simplest route. If you want to talk about collaboration, a website, a small product, or a macOS app, send a short brief and we can take it from there.
LinkedIn and GitHub stay here as supporting contact surfaces. GitHub is currently more of a public profile than a list of open public work.