There is an example in the udf manual to calculate the pressure force on a surface. You will need to follow same procedure. You can find the example under DEFINE_CG_MOTION
And like what mentioned, if the thread is not passed to the macro, DEFINE_ON_DEMAND for example, you will need to call the domain and the thread using
d = Get_Domain(1) ;
t = Lookup_Thread(d,ID);
Then you can loop over the faces, same way as in the example mentioned above.