First, please see the post: https://unmethours.com/question/21541/how-to-assigning-separate-fresh-air-handling-unit-with-fan-coil-unit-in-db/

Then go to the zone group and select any thermal zone. Select “Add Direct air” option and put it in your thermal zone.

After that go to “Add Loop” tab and select “Add Generic Air Loop”.

The last step is to connect the AHU to the fresh air intake and to the exhaust. You can put a heating and cooling coil, based on your design.