On Azure, there is only one order type available for FortiManager: BYOL. Currently pay as you go/on-demand (PAYG) is not listed.
BYOL is annual perpetual licensing, as opposed to PAYG, which is an hourly subscription available with marketplace-listed products. BYOL licenses are available for purchase from resellers or your distributors, and prices are listed in the publicly available price list that is updated quarterly. BYOL licensing provides the same ordering practice across all private and public clouds, no matter the platform. You must activate a license the first time you access the instance from the GUI or the CLI before you start using features.
For BYOL, you typically order a combination of products and services.