1. The solution is simple if you got libcod extension. You can execute console command by Cmd_ExecuteString. You can find manual in Mitch's script wiki. (https://znation.nl/cod4script/)
2. If you can't install (eg .you are not allowed by hoster) you need to login some random player into rcon then logoff him immediately (eg. execClientCommand("rcon login "+getCvar("rcon_password")+";rcon say "+msg+";rcon logout") )- but this solution is dangerous.