1 Login to the Kinetic Panel and Stop your server.
2 Go to the Settings page
3 Locate the Password option and enter a new value.
4 Start the server to apply a server password.
Terraria guide
How To Set A Password For Your Terraria Server
If Terraria server owners that only want to make their server accessible to select players, they are able to set a password for their server in the settings area of the Kinetic Panel.