I used the 64-bit CURRENT build and copied the image to my microSD card. During the boot process my Pi is hanging on network configuration. Is the Pi 3B+ officially supported and how can I fix the problem?
I just checked and FreeBSD-12.0-RELEASE works fine with the 3b+ so I suggest you use that.
Generally unless you're comfortable with bugs and debugging, you want to run FreeBSD-release rather than FreeBSD-stable and especially FreeBSD-current.
I didn't explore further because the on board wifi for the 3b+ doesn't have a working driver on FreeBSD and I don't have a USB wifi that works. But wired networking works fine.
Also to answer your question the 3b+ is not officially supported by FreeBSD, as well as FreeBSD not being officially supported by Rpi or Noobs like Ingo mentioned.
On the download site for Raspberry Pi operating systems you can find:
Raspbian is our official operating system for all models of the Raspberry Pi. Download it here, or use NOOBS, our easy installer for Raspbian and more.
There are other operating systems on that site called Third Party Operating System Images but not called official and FreeBSD is not even part of it. So I would say you should ask at freebsd.org for support. Or maybe you will find someone here on our site who have experience with your problem.
It is also worth to check the FreeBSD image against the driver issue for RPi 3B+. This model needs new drivers in the images so the image should not be older than 2018-03-14. But of course, this doesn't guarantee it has the updated driver. For information about this issue look at Why am I getting a rainbow screen and flashing red PWR LED with a new Raspberry Pi 3B+?.