Once you have the ISO, you must read this paper. The base ISO (RTM) is outdated. The paper explains that you must immediately apply the latest Cumulative Update (CU23 is the final stable build for Exchange 2016).
A: Use the 180-day trial from Microsoft Evaluation Center. It requires no license key during setup. --- Microsoft Exchange Server 2016 Iso Download
Microsoft Evaluation Center
Volume Licensing Service Center (VLSC)
Microsoft 365 Admin Center