I guess with English not your primary language, one has to ask what you mean with "LINK". Answers so far are telling you how to obtain Litchi App. The term Link means to communicate. So are you asking how to "Connect" (Use) the Litchi App with your Mini, or how to get the program?
If asking how to make it work, you run the App by itself, do not use Go Fly (DJI) or even start it, Just open the Litchi App first as you would the "Fly" App. It will "link/connect" as you are normally use to seeing with the Go Fly App.