SoftEther VPN User Forum

Because SoftEther VPN uses not single, but multiple protocols to connect.
There are OpenVPN, L2TP/IPSec, SSL, SSTP ...
If you are using native L2TP/IPSec connection on client,
offcourse you must supply valid PSK shared key, used for mutual data encryption.
SoftEther client uses own SoftEther VPN protocol,
so you need to connect only login and password.

You can set IPsec Pre-shared key in "IPSec/L2TP settings" of VPN server manager.