//create Tabulator on DOM element with id "example-table"
var table_export = new Tabulator("#export-export", {
 	height:600, 
 	data:tabledata_export,
 	placeholder:"No Data Available",
 	autoColumns:true,
 	autoColumnsDefinitions:[
        {field:"productid", headerFilter:true},
    ],
    rowClick:function(e, row){
		//e - the click event object
		//console.log(row.getData().productid);
		window.open("product_modify.php?productid="+row.getData().productid, "_blank");
    },
});

Us row.getData().productid to specify which cell to be returned in the row. (http://tabulator.info/docs/4.9/components#component-row)