max preview: how to populate "custom File" field via MXS

Hi ladies and gents, I'm hoping to improve a tool I've wrapped around the "Max Preview"/"Make Preview" dialog.

A couple versions ago (I'm in 2016 now) the Make Preview dialog gained a "custom File" field... I haven't found a way to use MXS to enable 'Custom File', populate the field just below it and then set filetype in the dropdown.

I've searched high and low in the docs, tried to find a way to get this dialogue to showproperties, etc but to no avail. Can someone point me in the right direction?

Do I need scratch write something through createPreview or getViewportDib. I'm hoping to use "Max Preview"/"Make Preview" since it does everything I need.

Thanks in advance for your help!