Under Administration->External Server->Syslog Targets... defind your SIEM server and appropiate port and protocol.
Then, define the syslog you require under Syslog Export Filter, attached the filter to the previously defined target.
Fore the filter, you'll lilely want to define this of a session log, then under the filter-tab, select Guest Access Request, you may want a second filter for RADIUS auth's as well.