Paypal pass custom variable to subscription button

I am trying to figure out how to pass custom variable when applies for a subscription for the webservice I have created;
So far my code on front end looks like this:

const createSubscription = (data, actions) => {
    return actions.subscription.create({
        'plan_id': 'P-XXXXXXXXXXX',
        "custom": "This is data from custom field",
        "custom_id": "This is data from custom_id field"
    });
};

But when my client is approved, IPN sends notification to my back end server, there is no field of custom_id or custom at all:

{
  mc_gross: '100.00',
  outstanding_balance: '0.00',
  period_type: ' Regular',
  next_payment_date: '03:00:00 Mar 17, 2021 PDT',
  protection_eligibility: 'Eligible',
  payment_cycle: 'Monthly',
  address_status: 'confirmed',
  tax: '0.00',
  payer_id: 'HXRWYDXHYFZN8',
  address_street: '1 Main St',
  payment_date: '10:45:14 Feb 17, 2021 PST',
  payment_status: 'Completed',
  product_name: 'Monthly Plan',
  charset: 'UTF-8',
  recurring_payment_id: 'I-F2C2YTJU2E1U',
  address_zip: '95131',
  first_name: 'John',
  mc_fee: '3.20',
  address_country_code: 'US',
  address_name: 'John Doe',
  notify_version: '3.9',
  amount_per_cycle: '100.00',
  payer_status: 'verified',
  currency_code: 'USD',
  business: 'sb-w7647v5112246@business.example.com',
  address_country: 'United States',
  address_city: 'San Jose',
  verify_sign: 'AwvhmU0vP2FlU28Jc6pBhDPQLGvKAuK1SbOO3rAWZruMDt8v8SJ',
  payer_email: 'sb-oettb5109886@personal.example.com',
  initial_payment_amount: '0.00',
  profile_status: 'Active',
  amount: '100.00',
  txn_id: '7M8958705N109551P',
  payment_type: 'instant',
  last_name: 'Doe',
  address_state: 'CA',
  receiver_email: 'sb-w7647v5112246@business.example.com',
  payment_fee: '3.20',
  receiver_id: '5CCP2BYDNEBQE',
  txn_type: 'recurring_payment',
  mc_currency: 'USD',
  residence_country: 'US',
  test_ipn: '1',
  transaction_subject: 'Monthly Plan',
  payment_gross: '100.00',
  shipping: '0.00',
  product_type: '1',
  time_created: '10:44:22 Feb 17, 2021 PST',
  ipn_track_id: 'bee6f9133b8f'
}

Is there any way to pass some extra information so that I could update record on my backend?

PS. Please pay attention, this is createSubscription not createOrder or something like that.