I just found this out myself. You need to set the value the same as one of the pre-value's values you defined. In the example below note the "Value" setting at the very bottom. This will cause Square to be selected by default.

   [
  {
    "label": "Set Image Crop",
    "description": "",
    "key": "class",
    "view": "radiobuttonlist",
    "applyTo": "cell",
    "prevalues": [
      {
        "label": "Square",
        "value": "ImgWrap SquareImg"
      },
      {
        "label": "Landscape",
        "value": "ImgWrap  LandscapeImg"
      },
      {
        "label": "Portrait",
        "value": "ImgWrap PortraitImg"
      }
    ],
    "value": "ImgWrap SquareImg"
  }
]