>     var Sheet = 'https://spreadsheets.google.com/feeds/list/SheetKey/od6/public/values?alt=json';
>
>     function callback(data){
>         var cells = data.feed.entry;
>     .
>     .
>     .

function GETURL(input) {
    var range = SpreadsheetApp.getActiveSheet().getRange(input);
    var url = /"(.*?)"/.exec(range.getFormulaR1C1())[1];
    return url;
}