How to get a value of wall shear stress for UDF
 

Hi, guys, I want to use the value of wall shear stress in my udf, I don't know how to define it. I see we can display the value of wall shear stress at the postprocessing in the software, FLUENT, but how could i use it during the calculating. i can't find a arguments name it.
Could anyone tell me how to define the a value of the wall shear stress which could be used in the udf, or how to calculate the equation M=integral(T*r*dA) in the software Fluent,M is the torque, T is the wall shear stress, r is the radius of the element to the centre of the face A, dA is the area of the element.
Thank you.