This is straightforward,
assemble(p*n[1]*ds(cylindermarker)
Where ds is a measure with subdomain markers marking the cylinder with a given tag.
I would suggest familiarizing yourself with dolfin, through the demos and for instance: https://zenodo.org/record/3564206#.X3Bcdvd4UlQ where I have code for computing drag and lift coefficients for the DFG benchmark.