You can find FortiClient technical document here:
 http://docs.fortinet.com/fclnt.html
 
 To save you time, the config you need to auto-connect is:
     <vpn>
         <options>
             <autoconnect_tunnel>your ssl connection name</autoconnect_tunnel>
 
 Then in the corresponding vpn connection, configure its user name, password, and make it keep-running:
 
                 <connection>
                     <name>your ssl connection name</name>
                     <username>user name here</username>
                     <ui>
                         <show_remember_password>1</show_remember_password>
                         <show_alwaysup>1</show_alwaysup>
                         <show_autoconnect>1</show_autoconnect>
                     </ui>
                     <password>password here</password>
                     <keep_running>1</keep_running>