Create new Prepopulated Data Variables

[BueDue]

Hi all, 

I want to create this Prepopulated Data Variables to store more district information. 

https://docs.whmcs.com/Additional_Domain_Fields

Example: 

$additionaldomainfields[".co.uk"][] = array(
"Name" => "Registered District",
"Type" => "dropdown",
"Options" => "{districts}",
); 

Please guide me how to create a new one !

 

Thanks & Best regards