Hello creativeworker,
with the IfPos-CoefficientFunction you can approximate the area force by a volume force
Code:
f_vol = IfPos( x-x0+epsilon, IfPos(x0-x+epsilon, 1, 0), 0),
where x0 is the cross section x-coordinate.
However, you will need a very high polynomial degree to "resolve" the small volume area.
If you have a mesh resolving the cross section (e.g. a structured grid) this approach will work with very small epsilon and low order polynomial degree.
Otherwise I would suggest to simply "cut" the geometry into two pieces and add them as one mesh, which should not be a huge problem with CSG.
Best
Michael