In a DOS prompt window, under the default installation folder of Fortclient (C:\Program Files\Fortinet\FortiClient), enter...

ipsec.exe Tunnel_name

...where Tunnel_name is the name of the IPSec tunnel you entered in the VPN tab under “Name”.

You can also enter ipsec.exe -d Tunnel_name. This will display the debug information of the VPN connection.

Use the following command to open the tunnel without showing DOS window:A good addition to remark would be to use the command:
start /B ipsec.exe -k

To close a VPN tunnel, you can enter the command ipsec.exe quit , to bring down the VPN Tunnel.