Retrieve ID from a custom object (Javascript)

Content

Hi! I'm trying to retrieve the ID value from a custom object, but I'm getting 'undefined'

var co = window.external.GetCustomObject('MATRIZ', 'MAT_MRFT'); 
​var idRegistro = co.GetCustomFieldByName('ID');

I've already tried this:

var idRegistro = co.GetCustomFieldByName('Id');

and

var idRegistro = co.ID;

and

var idRegistro = co.Id;

and

var idRegistro = co.GetCustomFieldByName('c$ID');

How can I get this value? Thanks!