How to define the square hole as inlet of the coolant

This is a cooling plate with a coolant flow channel inside. The square hole on the left is the coolant inlet. When I use “New Entity/group/create Groups” to create a group named inlet and then create a mesh, I can only define the entire plane where the square hole is located as the coolant inlet. How can I define only the square hole as the coolant inlet?

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did you ask the right question? Have a look at the Doc of your FEM solver.
As far as I remember, 2 bodies must be defined. Body 1 for the fluid and body 2 for the metal plate. Each body has its own surfaces. These are assigned boundary conditions in the FEM program.

with best regards

I have solved the Problem.The Method is wie your Suggestion.Thank you for your help.