@karlenigma You may have solved it by now but this is a working example from one of my scripts 👌

var suiteletURL = url.resolveScript({
    scriptId: 'customscript_shipping_cost_calc_sl',
    deploymentId: 'customdeploy_shipping_cost_calc_sl',
    params: {
        shipmethod: shipmethod,
        items: items.join(','),
        zip: zip,
    }
});
ctx.form.addButton({
    id: 'custpage_shipping_cost_calc',
    label: 'Shipping Cost',
    functionName: ";window.open('" + suiteletURL + "');"
});