Login to the Kinetic Panel.
Stop the server.
Open File Manager.
Navgite to tf > cfg > server.cfg
Locate sv_password "".
Change to be the desired password
Save the file.
Start the server.
Team Fortress 2 guide
How to add a password to your Team fortress 2 Server
This guide will show you how to make your Team Fortress 2 Server private by use of Passwords