Please select your admins in Front End PM PRO > Settings > Recipient > FEP Admins (more details in https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/only-admin/)
This list will be available when users send message. If you want your users can send message only to those admins please block them by “Role to Role Block” feature so that they can not send to any other role. Please see more details in https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/role-to-role-block/
Let me know if you have any doubt.