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>