form.html

<form class="newsletter-form top-margin-80 aw-mc-ajax-form" action="<?php echo admin_url( 'admin-ajax.php'); ?>" method="post">
	<input name="action" type="hidden" value="aw_chimp_subscribe">
	<div class="field-inside">
		<i class="fa fa-envelope-o"></i>
		<input type="text" name="email" class="email-field" placeholder="Type your email address...">
		<button type="submit">
			<span cLass="subscribe-text">
				<?php _e( 'Subscribe', 'appsworld' ); ?>
			</span>
			<span cLass="ajax-loader">
				<i class="fa-li fa fa-spinner fa-spin"></i>
			</span>
		</button>
	</div><!-- /.field-inside -->

	<div class="aw-mc-response"></div><!-- /#mailchimp-response -->
</form><!-- /.newsletter-form -->

function.php

<?php
/**
 * Mailchimp Email Subscription Handler
 *
 * @since 1.0.0
 * @version 1.0.0
 * @author Aminul Islam <[email protected]>
 */
function aw_chimp_subscribe() {
	// Mailchimp API and List ID from codestar option panel.
	$mc_API = 'Your MC API Key';
	$mc_LID = 'Your MC List ID';

	$srv = substr($settings['mailchimp_api'], -3);
	$subscribe_url = 'https://' . $srv . '.api.mailchimp.com/2.0/lists/subscribe';
	$email_struct = new StdClass();
	$email_struct->email = $_REQUEST['mmcemail'];
	$parameters = array(
	    'apikey'	 	=> $mc_API,
	    'id' 		=> $mc_LID,
	    'email' 		=> $email_struct,
	    'double_optin' 	=> false,
	    'send_welcome' 	=> true
	);
	$curl = curl_init($subscribe_url);
	curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
	curl_setopt($curl, CURLOPT_POST, true);
	curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($parameters));
	curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
	$response = curl_exec($curl);
	$response = json_decode($response, true);

	if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {

		if ( ! empty( $response['error'] ) ) {
			echo $response['error'];
		} else {
			echo 'Added! Thank you for subscribing.';
		}

		die();
	}
	else {
		wp_redirect( home_url() );
		exit();
	}
}

// Action it for ajax request
add_action( 'wp_ajax_nopriv_aw_chimp_subscribe', 'aw_chimp_subscribe' );
add_action( 'wp_ajax_aw_chimp_subscribe', 'aw_chimp_subscribe' );

scripts.js

$(".aw-mc-ajax-form").each(function () {
	var $this = $(this);

	// Newselleter Scripts
	$this.submit(function () {
		$this.find("button[type=\"submit\"]").addClass("clicked");
		var action = $(this).attr("action");
		$.ajax({
			url: action,
			type: "POST",
			data: {
				action: $this.find("input[name=\"action\"]").val(),
				mmcemail: $this.find("input[name=\"email\"]").val()
			},
			success: function success(data) {
				$this.find(".aw-mc-response").html(data).addClass("success").css("display", "block");
				$this.find("button[type=\"submit\"]").removeClass("clicked");
			},
			error: function error() {
				$this.find(".aw-mc-response").html("Sorry, an error occurred.").addClass("error").css("display", "block");
				$this.find("button[type=\"submit\"]").removeClass("clicked");
			}
		});
		return false;
	});
	
});

subscribe.php

<?php

// This file to handle form static html file

$subscribe_url = "https://us3.api.mailchimp.com/2.0/lists/subscribe";

$email_struct = new StdClass();

$email_struct->email = $_REQUEST['email'];

$parameters = array(
    'apikey'	 	=> 'MC_API',
    'id' 		=> 'MC_LIST',
    'email' 		=> $email_struct,
    'double_optin' 	=> false,
    'send_welcome' 	=> true
);

$curl = curl_init($subscribe_url);
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($parameters));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($curl);

$response = json_decode($response, true);


if (!empty($response['error'])) {
	echo $response['error'];
} else {
	echo "Thank you for subscribing.";
}