1.Go to /sys/server.cfg.
2.Open "server.cfg" in a text editor.
3.You can see a line called "sv_rcon", put after it your password.
(E.g sv_rcon themud123)
4.Done! You can run the dedicated and you can use the RCon there!
You can't put the password on the dedicated terminal screen.
You can by opening the server, and go to the "console" and write there "rcon_password <password>".