Posting custom data from contact form fields to Mailchimp list with API not working

Question

I'm trying to push the $POST_[values] from my form to my Mailchimp list of contacts, but the only two values I can get to be pushed are "email" and "status" (which is "subscribed").

I read all the documentation and I can't make it work with the provided merge fields.

HTML

<div class="input-box">
                    <span>First Name</span>
                    <input type="text" name="FNAME" value="FNAME" tabindex="1">
                </div>
                <div class="input-box">
                    <span>Last Name</span>
                    <input type="text" name="LNAME" value="LNAME" tabindex="2">
                </div>
                <div class="input-box">
                    <span>Email*</span>
                    <input type="email" name="email2" tabindex="3" required>
                </div>
                <div class="input-box">
                    <span>Company*</span>
                    <input type="text" tabindex="4" name="MMERGE9" value="MMERGE9" required>
                </div>
                <input id="submit-download" type="submit" name="submit2" tabindex="5" disabled></input>

PHP

include('./mailchimp/MailChimp.php'); 
$MailChimp = new MailChimp('XXXX');
$list_id = 'XXX';
$email = $_POST['email2'];
$MERGE9 = $_POST['MMERGE9'];
$result = $MailChimp->post("lists/$list_id/members", [
                'email_address' => $email,
                'status'        => 'subscribed',
                'MERGE9'        => $MERGE9,

I tried all variations provided by Mailchimp: MERGE9, MMERGE9, |MERGE9|

I use this library: https://github.com/drewm/mailchimp-api

I also tried to list the merge fields using the curl command but I can't get it to work:

https://mailchimp.com/developer/api/marketing/list-merges/

Answer 1

Note: This question focuses on using a custom HTML form to post new members to a MailChimp list using the MC API; code relating to validation, security, spam/reCaptcha and other important considerations has been omitted. I don't recommend using this code on its own in production.

Original issue: When posting a new 'member' to a MC 'list_id' with their API [using post("lists/$list_id/members")], only two fields, 'status' and 'email_address', are stored, ignoring all the merge_fields I wish to also store.

Solution: 'merge_fields' have to be submitted as an array using the MC API.

In order to solve my issue, I found it useful to experiment a bit directly with the API using curl using GET, as the mailchimp-api library by drewm is great but it hides a bit of the inner workings.

PHP

<?php
include './../mailchimp/MailChimp.php'; // update this \path\to MailChimp.php from drewm mailchimp library (https://github.com/drewm/mailchimp-api);
use \DrewM\MailChimp\MailChimp; // do not change/edit this line;
if(isset($_POST['submit'])) { //edit as needed the name of the submit field in your form;
    $MailChimp = new MailChimp($key); //update $key with your MailChimp API key; should look like 3856ojd298g5q82c2213453a98172346-xx22; can be found under >>Top Right Corner dropdown (displaying your name)>>Account>>Extras>>API Keys>>Create a key [keep this key private as gives partial API access to your account]
    $email = $_POST['email']; // update field name as needed;
    $fname = $_POST['FNAME']; // update field name as needed;
    $lname = $_POST['LNAME']; // update field name as needed;
    $company = $_POST['COMPANY']; // update field name as needed;
    $result = $MailChimp->post("lists/$list_id/members", [      // update $list_id with your list_id; should look like this 'js83js93jm'; can be found under MailChimp.com>>Audience>>Audience Dashboard>>Manage Audience dropdown>>Settings>>Audience name and defaults>>Audience ID
                'email_address' => $email, // this field is a MC API default, don't edit;
                'merge_fields' => [
                    'FNAME' => $fname,
                    'LNAME' => $lname,
                    'MMERGE9' => $company,
                    ],
                'status'        => 'subscribed', // this field is a MC API default, don't edit, although you could possibly remove it if you don't want to add the subscribed status at this point (verify in the docs);
                
            ]);
?>