@Micke1101 great find!
I've apparently never read that far into the file. Such a big one, with no comments.. brutal.
You would want to define default as an empty string as well, like your 3223 one:
$default = isset($config) ? $config:'';
Or if you were particularly vicious, just embed one ternary inside the other.. but don't. Lol.
Then, to add the email domain, actually edit the JSON field definition by hand, or edit the configuration to add the widget to the admin form.
This might well better as a pull request, I'm sure other forms text boxes could use a default value as well.