Currently there is no option to specify the Domain Controllers site when setting up a new DC in a new forest. (There is also no samba-tool command to move a DC to another site.)
Created attachment 10057 [details] proposed patch for 4.1, cherry-picked from master
Assigning to Karolin for inclusion into 4.1.NEXT
(In reply to comment #2) > Assigning to Karolin for inclusion into 4.1.NEXT Pushed to autobuild-v4-1-test.
Pushed to v4-1-test. Closing out bug report. Thanks!