Well the button for that isn't going to be directly on the phone. It will be in your facebook. So if you are trying to make it be on the phone itself, that won't work. However if you are trying to access it on facebook, on your phone, you can try the following....
How to share a live video via Facebook Live for Android.
1. While in your Facebook app, tap “What’s on your mind?” (at the top of your News Feed).
2. From this screen tap the "Live Video" icon (it looks like the head of a person with signal
broadcasting around their head).
3. Now, if you wish to, you can enter a "Discription" for the video.
4. Next you choose the "Audience" for your video (Public, Friends, etc.).
5. After you have chosen, now tap "Go Live". You have now started your live broadcast.
6. When you want to end the broadcast, just tap "Finish".
Take Note: Remember, the maximum time you can do a video for is 30 minutes.