I found solution using bootstrap popup and mailchimp shortcode.
You need to install mailchimp plugin.
https://wordpress.org/plugins/mailchimp-for-wp/
Step 1 : Customise menu item
function nav_replace_wpse_189788( $item_output, $item, $depth, $args ) {
//var_dump($item_output, $item);
if( $args->theme_location != 'main_menu' ){
return $item_output;
}
if ( $item->menu_order == '3' ) {
$item_output .= '<li class="moblie-nav menu-item menu-item-type-custom menu-item-object-custom last-menu-item">';
$item_output .= '<a href="https://wordpress.stackexchange.com/questions/338839/JavaScript:Void(0)" data-toggle="modal" data-target="#myModal">Subscribe</a>';
$item_output .= '</li>';
}
return $item_output;
}
add_filter('walker_nav_menu_start_el','nav_replace_wpse_189788',10,4);
Step 2 : Add popup with mailchimp shordcode
<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog"
aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">
<img src="https://wordpress.stackexchange.com/questions/338839/images/close-icon.png">
</button>
</div>
<div class="modal-body">
<div class="exp_login_wrap">
<h3>Subscribe</h3>
<?php echo do_shortcode( '[mc4wp_form id="26125"]' );?>
</div>
</div>
</div><!-- /.modal-content -->
</div><!-- /.modal-dialog -->
</div><!-- /.modal -->