How to make customFields qiwi node js?

Question

Node.js

There is a code

module.exports = {
  tag: ["buycoinscsc"],
  button: ["buycoins"],
  func: async(context, { vk, _user, game, util, qiwiApi }) => {

let sum = await context.question({message:`Введите кол-во коинов которое хотите купить`});
if (sum < 10) {return context.send(`Минимальная покупка 10 коинов`)}
let sumpay = sum / 10 ;
let rand = util.random(1,100000000);
const publicKey = '48e7qUxn9T7RyYE1MVZswX1FRSbE6iyCj2gCRwwF3Dnh5XrasNTx3BGPiMsyXQFNKQhvukniQG8RTVhYm3iP6DtLsHceheQcDZdrgEvThFdmjBpa9xtsqJ5wwcd2vZDqAZSQbja24fkYr9bhqjebwCY7CJW16DHbuCi7qCbXSKomc9kc946FbNeZVayD4';

const params = {
    publicKey,
    amount: sumpay,
    billId: rand,
    successUrl: 'https://vk.me/csgowheel',
    comment: _user.uid,
    customFields: {themeCode: 'Mykhayl-MZbK1PNHfK'}
};

const link = qiwiApi.createPaymentForm(params);
let url = (await vk.get()._vk.api.utils.getShortLink({ url: `${link}`})).short_url
await context.send(`Перейдите по ссылке и оплатите ${sumpay} руб за ${sum} CS coin 
Ссылка: ${url}`);
   }
};

however, there is an error in creating the form!