Amortization is captured in principal and interest (P&I) payments. For example with a 6-months fixed interest only (IO) period selected on the Loan page, the first P&I payment will be after the IO period ends, with the First Principal and Interest Payment Due Date. If the first payments are amortizing, then that would be a P&I payment and you can remove the IO period.
However, if the first payments are amortizing, then that would be a P&I payment and you can remove the IO period of 6-months.