laconic.com/src/pages/api/mailchimpcontact.js

const mailchimp = require('@mailchimp/mailchimp_marketing')

mailchimp.setConfig({
  apiKey: process.env.MAILCHIMP_API_KEY,
  server: 'us8'
})

export default async function handler(req, res) {
  const body = JSON.parse(req.body)
  const email = body.email
  const inquiry = body.inquiry
  const message = body.message

  // eslint-disable-next-line no-console
  try {
    const response = await mailchimp.lists.addListMember('5b23f4626d', {
      email_address: email, // MERGE0
      status: 'subscribed',
      merge_fields: {
        INQUIRY: inquiry, // MERGE3
        MESSAGE: message // MERGE4
      }
    })
    res.status(200).json({ response })
  } catch (err) {
    res.status(500).json(err)
  }
}