Ok...

I need to define an area on a surface where a load is applied (for my static structural analysis)

E.g., on one face of a cube I need a 5x5mm area where a load is applied.

What I do: sketch a 5x5 square and extrude/cut it 0.0001mm... to mark the area on the model.

I don't think it's right but how can I do it properly?

Thank you.