I no longer have the file in that state as I simply worked on a copy layer with the regular content aware fill.
But I can describe you the steps.
1. Flatten image
2. Make layer
3. create a crop frame reaching outside the image borders. "Content aware fill" is checked in the crop tool's control panel
4. Hit enter to crop.
5. Result: the original image plus the background in black where you extended the image via crop tool. If you uncheck "delete cropped pixels" you get a transparent area where you wanted the extended image area instead of the content aware fill.