Welcome to the Community,
You can use the ESXi Hypervisor license key you got with registering for the HP OEM build too. The HP version builds on the original VMware version with additional vendor specific drivers and tools already included.
When you install ESXi it will always run in a 60-day evaluation mode which allows you to evaluate all features of the Enterprise Plus edition. As soon as you enter the free license key, advanced features will be locked down and you will have the 60-day evaluation mode changes to unlimited use.
André