The first time you log into your system an information notice will be displayed, note the lightbulb icon in the panel. Mousing over the icon will reveal a menu and after selecting it, the dialog shown above will be displayed.
Clicking the Run this action now button will open a terminal window. Enter your login password at the Passphrase prompt and the encryption passphrase will be displayed.
If you do not save your passphrase now you can do so later by using the command