Substitution tax code in ME21N

We want to be able to substitute the tax code when a user is making a purchase order (tcode ME21N). More in detail: when a person enters a certain cost center we want the tax code automatically to be filled with a certain tax code value.

The substitution enviroment (tcode GGB1..4) does not work with ME21N and also we cannot find a suitable user-exit.

Any suggestions?