I'm new to FreeCAD (and have never used other CAD software), but I think in your case, you can click the face on Part Design view, then click New Sketch. It wil

I'm new to FreeCAD (and have never used other CAD software), but I think in your case, you can click the face on Part Design view, then click New Sketch. It will auto-select that plane with placement automatically set to the selected plane. The UI/UX wasn't very intuitive, though, as it won't work if you click New Sketch first, then select a plane.