How to set Join Before Host Setting

Initially, do the following (this is a one-time setting),
Go to settings and search for 
Allow participants to join before host in the search box.
Turn on the settling.
Check the
Participants can join check box.
Set the desired time by selecting it from the dropdown. It's recommended to keep the time either 
5 minutes before start time or 
10 minutes before start time. Do not set it to
anytime.
Check the settings below every time you schedule a meeting.
Scroll down until you find 
Options
Click 
Show 
See if the 
Allow participants to join check box is checked. If not, check it.
Adjust the time if needed.