On version 4 of ckeditor there was the possibility to display a dialog box when creating a custom plugin, but on version 5 it is not possible ?
Ckeditor v4
CKEDITOR.dialog.add('', function (editor) {
var dialog = {
};
return dialog;
});
There's no UI component for dialog box in ckeditor 5 and they don't plan to create any.
It's recommended to use contextual 'balloons' instead.