This command adds a heir to your country - or the country with the specified country tag (if you specify a country tag). If a heir already exists, this command will replace the heir with a new heir.
add_heir [country tag]
|Country Tag||Optional. The tag of the country you wish to add a heir to. If not specified, the heir will be added to the country you are currently playing as.|
As no country tag is specified in the above example, a new heir would be added to the country you are currently playing as, replacing any existing heir.
As SYR (Syria's country tag) is specified, a new heir would be added to Syria upon execution of the above console command.