Thank you for your purchase.
please see https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/only-admin/ for only admin setup. If more than one admin added then dropdown will show. Please go to Dashboard > Front End PM PRO > Settings > Recipient

If you set “Max recipients” as 0 then users will not be able to send message to other users and to box will be hidden so that users will not be able to type.

Announcement can be added from Dashboard > Front End PM PRO > New Announcement. Also if you want to add from front-end please check Dashboard > Front End PM PRO > Settings > Security > Add Announcement