Few days after the release of the new Windows 10 build (1803, also known as April Update), the ADMX files to manage GPO settings for Windows 10 1803 are now available for download here https://www.microsoft.com/en-us/download/details.aspx?id=56880

NOTE for some reason, the files are not extracted to the defined target folder when running the MSI package (default “C:\Program Files (x86)\Microsoft Group Policy\Windows 10 April 2018 Update (1803)\”) but to C:\PolicyDefinitions. Even if you define a different target folder.