Hi Zain,
You have to create a condition in SD for with holding tax and this tax condition you have to mapped in your SD pricing procedure.
After that you have to assign this tax condition to w.h.tax type in the following node FA-Withholding tax-Extended withholding tax-calculation- Withholding tax type - assign condition type to withholding tax type.
Based on this system will post w.h.tax at billing.
Regards,
Mukthar