Allow users to specify custom position for the dialog

I assume my problem is syntax and nothing else; the outcome is that I fail to position the iframe as I'd like. Would you provide an example of defining 'position' in customDialogOptions? Or does it go elsewhere?

Here's the failing code:

        var modalOptions = {
          url: url,
          autoFit: false,
          draggable: false,
          width: 700,
          height: 500,
          customDialogOptions:  ({
            // This does work.
            modal: false,
            // But this doesn't.
            position: 'top',
          }),
        };

I'd be even happier with an example of an array position (e.g. [350,100]) .

Many thanks