Enter it as an additional cost and turn on the add to ad spend switch.
That spend then behaves like Meta or Google spend: it affects ROAS, True ROAS and the rest of the advertising metrics. You can enter it as recurring or by period, and in dollars if needed.
The reverse matters too: if an additional cost is not advertising —rent, salaries, your accountant— that switch must stay off, or it distorts every advertising metric.