Hi,
Standard SAP will copy the tax code from PO item to MIRO item automatically. If its not getting copied, please check whether any development is done which is altering the standard behavior.
If no development is done and still tax code is not getting copied from PO item to MIRO item, please revert back with your SAP release with EHP.
The tax code at MIRO header will be from configuration in OMR2.
Regards,
AKPT