Defining a coastal boundary as one of my boundary conditions using FEFLOW

I am trying to model salt water intrusion along a coastline. The FEFLOW book and user manual suggest using a type 1 (Dirichlet-type) BC.

Do I then just impose a mass concentration along this boundary?

I am also in the process of learning FEFLOW so please break steps down as much as you can.

Thank you!

As a first step, you may have a look at the 'Henry' benchmark that comes with FEFLOW. It'll give you an idea of how to choose the BCs for the sea.