Tax allocation in Accounts Payable

Hi

We have some situations where the entire amount of an invoice is to be allocated to the tax account. How is this handled using a tax ex. code and a tax rate area which would allocate the entire amount to the tax account as follows as the user does not want to perform a manual amount allocation on the GL distribution screen. Reason being when the tax codes are used the tax amounts appear in the  P0018  and the request is for this functionality should be preserved

scenario

Invoice amount is 100. of which the entire amount is a tax amount.