Microsoft MVP, Julie Turner, wrote (2017) a great article on how to launch a Microsoft Flow from client-side code.
“The requirement seemed deceptively simple… and it was, somewhat… simple. I’ll start with showing you how very simple it is to launch a Microsoft Flow (“flow”) from your client-side code hosted, well… wherever. I will give you this caveat, launching a flow this way requires no authentication. The URL is entirely obscure, but if you’re concerned that the flow you’re starting does something you only want to allow authenticated users in your organization to do then you may want to rethink this. As with all security issue you need to assess and balance security with risk. I suppose that’s true of life too.”
– Julie Turner