problem regarding offset tax receivables

Hello.

I got problem to set up offset tax in AR (R11). I need the following result when enter the transaction:

Debit - account receivables - 100
Credit - income account in 3 series - 100
Debit - income tax in 8 series - 20
Credit - withhold tax payable in 2 series- 20

I have tried to enter a tax group as one note at metalink refered to:

A tax group must be created with the VAT tax code used for invoicing purposes and with the Offset tax code assigned to it.
To do it, the following instruction should be followed:
  1) From a Receivables responsibility ;
  2) Navigation path: Setup --> Tax --> Groups ;