Microsoft's Windows Server team released a preview of the next Windows Server product, Windows Server 2019, yesterday to the public.
Windows Server 2019 Preview is available for any participant of the Windows Insider program but the post on Microsoft's Windows Server website is unfortunately a bit vague when it comes to actual downloads of the preview version of Windows Server.
The very first thing you need to do is register an organization account or a personal account; Registering in this case means that you join the Windows Insider Program with the account so that you may download and use Insider Builds.
Once you have done that head over to the Windows Insider Preview Server download page and select "Windows Server vNext LTSC PReview" under available downloads. You can download localized versions of the Server 2019 Preview ISO and have 24 hours to download the 4.2 Gigabyte ISO image from Microsoft.
Microsoft plans to release the final version of Windows Server 2019 in the second half of the calendar year 2018. The company plans to release System Center 2019 as well which will support Windows Server 2019.