Hello,

I ran into this as well and I believe all you need to do is, wherever you are setting the Parent field in your script, use "parent.value" rather than just "parent". See below where I am setting the short description to my 'testVal' variable added as a parameter when instantiating the modal form:

function cancelTicket() {
    var d = new GlideModalForm('Create New incident', 'incident');
    d.setOnloadCallback(populateModal);
    d.addParm('testVal', 'test');
    d.render();

    function populateModal() {
        //Access GlideModal g_form
        var d_form = window.frames["dialog_frame"].g_form;
        d_form.setValue('short_description', testVal.value);
    }
}

I hope this helps!