Please go to Dashboard > Front End PM PRO > Settings > General > Show Directory
Uncheck this and save. This will show Directory only to admins.
To pre-populate “To” field you can use https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_message_to/. It will show a link and when any user will click that link then “To” field will be auto populated.