const stripe = require("stripe")("api_key");

const CUSTOMER = "";
const PLAN = "";
const quantity = 1;
const startTimeInSecondsSinceEpoch = 0;
const billingCycleAnchorInSecondsSinceEpoch = 0;

async function foo() {
  const subscription = await stripe.subscriptions.create({
    customer: CUSTOMER,
    items: [{ plan: PLAN, quantity }],
    backdate_start_date: startTimeInSecondsSinceEpoch,
    billing_cycle_anchor: billingCycleAnchorInSecondsSinceEpoch,
    proration_behavior: "none",
  });
  console.log(subscription.id);
}

foo();