syslog messages from your gateways/mgmt to a syslog server, or firewall logs to a syslog server? If its the gateway/mgmt to a syslog sever the string is below.
add syslog log-remote-address <target server> level <level>
If its traffic logs, then per the SK Val posted you would use a syntax like below specifying syslog as the format.
cp_log_export add name <name> [domain-server <domain-server>] target-server <target-server IP/host name> target-port <target-port> protocol <(udp|tcp)> format <(syslog)|(cef)|(splunk)|(logrhythm)|(generic)> [optional arguments]
For you I would assume the string to look something like this
cp_log_export add name SYSLOG-EXPORT-TO-SIEM target-server 10.10.10.10 target-port 514 protocol udp format syslog
