[Corinthian]

I have never done it but according to the manual:

2.5.1 #start
By creating at least one start location for each
player, every player will start at one of these
locations. If #start provinces are set, nations will
start at these locations unless there are more
nations than #start provinces. If there are more
nations than #start provinces, the extra nations
will start in eligible random locations.
If a province is set as a #start province but its
terrain mask includes the value 512 (nostart), the
nostart will override the #start command and no
nation will start there.
If no #start provinces are set, all provinces are
available as random starting locations unless set
nonstartable with the #nostart command or in
the map editor.

Keep in mind that some nations must start in the water or in coastal provinces witch can cause problems for these commands.