How to Enable Forms Pro Preview?

Our organization isn't able to access the Forms Pro Preview--when any of us try to sign up, we get an error message stating "Forms Pro Preview is not yet available for your work or school." I'm sure there must be a simple reason for this, but I've run out of places to look for a solution. My organization is a Commercial customer and we have E3 licenses, so as far as I'm aware, we've fulfilled what the Forms Pro opt-in documentation asks a user to do.

Where I'm stuck is this bit about "If you get an error while signing up for Forms Pro preview, contact your administrator to enable the Forms Pro preview in your tenant." I am the administrator, and I haven't seen anything obvious in our Admin Center that would indicate what I need to do to turn that on. I don't see it in Services and Add-Ins, I don't see it as a free product in Purchase Services, I don't see it as a Purchased Service...where is this?