Tax in Obligation

How can I enter tax in obligation invoice in RTM/RMS?

Without tax information, invoice should be entered into RMS/RTM for ALC, and again manualy entered in SAP. I want to avoid entering the same document twice.

Consider this example:

We have PO with total merch. cost of 1000€ (+Tax), and additional invoice for transport cost of 100€ (+tax).

When entering merch. invoice to ReIM, we are able to enter all information regarding the invocie: total cost (1000€) and total tax (250€).

When entering obligation, I can only enter transport cost without VAT: 1000€.

However - in order to post the obligation invoice to ReIM and to SAP after that, I need the Tax information - amount and tax code.