It’s in the upper right corner of the window
in the following menu
It’s in the top bar of the window
Under the Junior Mode heading
This will open its sub-menu
This will reveal more options at the top of the window
That’s at the top of the window
The Always (also) Manually is the most important to check
Account Settings window will open
It’s in the menu on the left side of the window
That’s at the very top of this page
Under the Message Composition Default Options heading
Under the If both, Enigmail and S/MIME encryption are possible, then: heading
and set the rest to your liking
in the previous window