Send Messages & SMS from Web & Email with a URL Trick
Mac and iOS users can start iMessage conversations and send text messages from the web, email, or anywhere else a link can be clicked, by using a custom URL to launch the Messages app. Using this you can start an iMessage chat to any other iMessage user or send them an SMS through the Messages app (from an iPhone, or assuming SMS relay is setup on the Mac). This is a neat little trick that is quite similar to starting a FaceTime call from a URL or the web and it can be useful for staff directories, internal webpages, emails, HTML signatures, or even if you just want to offer a simple method of contact through a general webpage.