Here’s the trick:

1: Go to Publishing Tools on your Facebook Page

2: Click Videos

3: Click +Live

4: Set up your video options and the schedule the live video

There will be a post on the page telling your fans that you’re going to go live, but there’s no way to boost it.
5: Click the share button and share it to yourself in messenger. When you click the link, that’s the link you want to use in the next step…

6: Make a post on the Facebook page explaining that you’re going to go live, and that people can get a reminder using the link in the comments. Publish the post and then paste the link from step 5 into the comments.
When you hit enter, there will be a button for people to get a reminder:
Boosting this post will mean people can get their reminder for when you go live, and your live audience will be instantly bigger! Enjoy!