Create a variable named ConfigFile.
Default value is server.cfg.
In "Commandline Parameters Options" tab
Check:
Admin access
Sub admin access
Reseller access
User access
Value is required
Item Type: Textbox
Label: Server Config:
Deny Characters: ", +exec, +set
In the game's commandline tab > predefined commandlines add this:
Description: Advanced
Commandline: +set fs_savepath $[service.UserDirectory] +set dedicated 2 +set net_ip $[service.IpAddress] +set net_port $[service.GamePort] +set sv_maxclients $[service.Slots] +set sv_punkbuster "1" +exec "![ConfigFile]" +map_rotate
For a working example you can check the Enemy Territory config.