I don't think this has been posted yet, so I'll post it.
Open base\config\server\prefs.cs
Line 3 and 4:
$Pref::Server::AutoAdminList = " ";
$Pref::Server::AutoSuperAdminList = " ";
Within these quotations, put the BL_id of who you want to auto Admin or auto Super Admin (Separated by a single space.)
[Edit] - Make sure your game is shut down before you edit the prefs.cs file, otherwise when you close the game it'll reset to the setting at point of game start. If you want to edit with game running, just put the commands in the game console.