In the web interface of Tvheadend, I just click the Details icon in the far left column of the program and then press the Play program button in the dialog. From Firefox, I can then open the resulting document (which mostly consist of a URL) using a player such as mpv or VLC. I can also save it and then open the stream by Drag&Drop, e. g. That's with Tvheadend 4.0.9, which is a little older, I think. It runs remotely, but I don't think that should matter.