pfSense

It is not possible to use Shared Secret by L2TP client,
no such field in WebGUI

http://mpd.sourceforge.net/doc5/mpd48.html#48:

set l2tp secret secret
 Sets the L2TP tunnel secret. Used to authenticate tunnel connection and encrypt important control packets avpairs. For server side, 
 only one unique secret supported for every pair of listening IP (set l2tp self ...) and peer ip (set l2tp peer ...). If several 
 secrets defined, only the first matching will be used for all incoming connections.

 NOTE: This options is not related with usual PPP authentication. Windows client does not support tunnel authentication.

see also #10527