Thanks for your response, I will continue this threat on a pull request.
Thanks for the advice of applying the force as Newman you have a right, I found that by myself, but it’s good to have a second opinion.
Sadly the FenicsX doesn’t have a PointSource, so I use another approach of modifying the Function vector directly which works for a single point, but I still have problems mapping all loads over the interface. Further discussion in the pull request :).