As one of the many people who decided to buy OrbusVR on Oculus Store rather than Steam I sadly have no way of assigning a Push To Talk button, this is obviously quite annoying as i would like to choose when to broadcast my mic and due to this it has made me a bit of a mute during my time in-game.
I had decided to look into this and had thought that maybe running the game through Steam would allow me to use the advanced settings in SteamVR to bind a key, sadly this did not work as the steam overlay was not displayed upon pressing the menu button on the left controller and the PTT icon wasn’t appearing when using the button keybind which I had already previously tested in other games.
I then had a thought to try ReVive by using the injection option from the taskbar and booting it that way and to my surprise my push to talk keybind finally worked as confirmed by a friendly nearby player who helped to test this.
However the problem with this method is that it gives a horrid headache inducing side effect that makes the world warp slightly as you turn your head… almost like a fish eye lens.
I know alot of people have been trying to get this to work so i thought that although not practical, my findings may help to create a work around until that faithful day that a PTT bind is added for Oculus Rift. Although I do understand its not the top of priorities at the moment).
If any of the Game Devs read this I would like to say, great job on the game its great to see MMORPGs coming to VR it has certainly been one of my favorite experiences to date. 
P.s I did also try the AutoHotKey method but its too unreliable for PTT