Fortigate / FA VPN user names

Hello

I am having a problem with Fortigate 100D / Firewall Analyser logs for VPN.

It shows hosts but it doesn't show user names. they are all "unknown" to Firewall Analyzer

This is the log example I have:

<189>date=2015-09-03 time=11:30:41 devname=FGT-1 devid=FG100D3G14803136 logid=0000000013 type=traffic subtype=forward level=notice vd=root srcip=192.168.111.29 srcport=61627 srcintf="port1" dstip=10.23.18.74 dstport=50100 dstintf="man-vpn-sap-ph1" poluuid=316d527c-c6a9-51e4-c42c-b93e17e96655 sessionid=71375901 action=close policyid=28 dstcountry="Reserved" srccountry="Reserved" trandisp=noop service="tcp/50100" proto=6 duration=35 sentbyte=2543 rcvdbyte=1317 sentpkt=6 rcvdpkt=4 vpn="man-vpn-sap-ph1" vpntype=ipsec-static

How can I add "username" so I can identify users that are using VPN?

Regards