From Console:

Tutorial Video. Step by step below;

1

First Step

2

Second Step

Select your Server

3

Third Step

In the Console, Type whitelist [value]. If you’d like whitelist to be enabled, change [value] to on and if you’d like it disabled, change [value] to off.

4

Fourth Step

To add/remove a player from whitelist, In the Console type whitelist [add]/[remove] [username], Where [username] is the Game Tag of the player you want to whitelist.

There you go! You’ve enabled/disabled whitelist for your server and added/removed a player from the whitelist!
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