test.js

document.getElementById("mainCanvas").addEventListener("drop", function (event) {
    event.preventDefault();
    event.stopPropagation();

    var fieldName = event.dataTransfer.getData("fieldname");
    var mergeField = new TXTextControl.MergeField;
    mergeField.name = fieldName;
    mergeField.text = "«" + fieldName + "»";
    TXTextControl.addMergeField(mergeField);
});