Send form data to mailchimp Raw

sendToMailchimp.php

<?php
// ==========================================================================
// Mailchimp
// ==========================================================================
//
// Send user to mailchimp
// --------------------------------------------------------------------------
public function sendToMailchimp(Request $request)
{
    // customer info
    $first_name = $request->first_name;
    $last_name = $request->last_name;
    $email = $request->email;
    // setup
    $api_key = "apikey [APIKEY]";
    $list_id = "[LISTID]";
    $api_endpoint = "https://[urlprefix].api.mailchimp.com/3.0/lists/{$list_id}/members";
    // client
    $client = new \GuzzleHttp\Client();
    $res = $client->request('POST', $api_endpoint, [
        'headers'  => [
            'Authorization' => $api_key,
            'content-type' => 'application/json',
        ],
        'json' => [
            'email_address' => $email,
            'status' => 'subscribed',
            'merge_fields' => [
                'FNAME' => $first_name,
                'LNAME' => $last_name,
            ]
        ]
    ]);
    if($res->getStatusCode() == 200){
        $message = "Successfully Subscribed";
    } else {
        $message = "There was an error adding you to our newsletter list";
    }
    return redirect()->back();
}