To enable the whitelist on your server you will have to either have specified it during your order (exclusive join) or modify the options of your server from the customer area if you forgot to activate the whitelist when ordering.
Then you will have to retrieve the ID of the players to add to the whitelist.
For epic games players you will have to launch the single player game and type Cheat listplayers in the console (press TAB twice to deploy the console fully).
For steam players this is steamID64. If you don't know it you can use this tool: https://steamidfinder.com/
Now edit the PlayersExclusiveJoinList.txt file from control panel> config files.
Add 1 ID per line, save and restart the server.