Unfortunately at this time there is no way to password a server, the game developers do have it high on their list of priorities though.
However, you can whitelist the server so that only those whitelisted can enter, to set this up just follow the below
1. In your game panel navigate to configuration files > whitelist.txt
2. Inside this file enter (on separate lines) the steam64id's of anyone you want to be able to enter the server (you can get your steam 64 ids' from here https://steamid.io/)
3. save the file then reboot the server
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