Update withholding tax code from abap exit/BADI

Hi all ,

This is my first post in SCN .Kindly ignore incase of any mistakes.

I have a requirement wherein i have to change the withholding tax code based on Expense account and vendor from backend from tcode F-43.

The tax code is to be picked up from a z table. I am not able to find any suitable exit/BADI to update teh BADI.

I am unable to change it through Substitution as withholding tax code data is not available at substitution exit.

Kindly help me out .

Regards

Suman