MsgL0ck is a simple iMessage app that lets you send text messages that requires the receiver to authenticate with the authentication method they have setup on their device. Like Touch ID, Face ID or Passcode. The receiver of the message needs to have the app installed as well.
Important to know is that the message you send is not encrypted. It is just prevented to show up before the user have authenticate. The purpose of the app is to hide text messages from the regular feed. That will prevent someone from taking your phone in unlocked state and read you texts directly.
Up to six family members will be able to use this app with Family Sharing enabled.