Substitute Tax code field (MWSKZ) when posting GR through MIGO

Hi, is it possible to subsitute tax code field MWSKZ when posting a GR through MIRO from PO by using substitutions (OBBH) and abap code in user exit ZRGGBR000? I am trying to populate tax code field in the GR financial doc by pulling it from PO, I created a substitution with some prerequesites like transaction code equal to MIGO or MKPF and trigger user exit form U90X with abap code, but I ahven't been able to do it. If this is not feasible, is there any other way to do this? Please advise. thanks, Juan F Vargas