Set an administrator password by editing the GameUserSettings.ini file from the panel > config files or by FTP Saved/Config/WindowsServer/GameUserSettings.ini

The line to edit is ServerAdminPassword=

For example, ServerAdminPassword=adminpassword

Save.

To add an administrator, edit the \Saved\AllowedCheaterSteamIDs.txt file from the Panel > Config Files or using File Manager or FTP.

Add your steamID64 to a new line (1 steamID64 per line and per admin).

Save and restart the server.

Connect to your server, open the console (TAB or whatever you have bound for it) and type:

enablecheats adminpassword

Replace adminpassword by the passord you chose.