After months of speculation, the Android Oreo beta program for the ZTE Axon 7 has opened up. The first come, first serve program is open to owners of the 2016 device with the US model A2017U. You can attempt to sign up here.
In the post announcing the program, ZTE indicated that as they get further into the beta program, more people will be added to it. So, for now, if you don’t get picked, don’t fret too much. You could get it later and frankly, being on the early builds of any beta is risky.