I have been created custom search form in my WordPress theme. She looks like:

<form role="search" class="form" method="get" action="<?php echo home_url('/');?>">
  <input type="search" id="keyword" class="inp-search" onclick="showNewTag()" oninput="searchTags()" onkeyup="fetch()" placeholder="search by tag" value="<?php echo get_search_query() ?>" name="s" title="Search" />

And i realized custom AJAX search by POST title. But my task is search by tags. I don’t know how i can do that. My search by post title located in functions.php and looks like:

add_action( 'wp_footer', 'ajax_fetch' );
function ajax_fetch() {
?>
<script type="text/javascript">
  function fetch(){

    jQuery.ajax({
    url: '<?php echo admin_url('admin-ajax.php'); ?>',
    type: 'post',
    data: { action: 'data_fetch', keyword: jQuery('#keyword').val() },
    success: function(data) {
        jQuery('#grid').html( data );
    }
});

}
</script>

<?php
}

 add_action('wp_ajax_data_fetch' , 'data_fetch');
 add_action('wp_ajax_nopriv_data_fetch','data_fetch');
 function data_fetch(){

   $the_query = new WP_Query( array( 'posts_per_page' => -1, 's' => 
 esc_attr( $_POST['keyword'] ), 'post_type' => 'post' ) );
if( $the_query->have_posts() ) :
    while( $the_query->have_posts() ): $the_query->the_post(); 

        if(get_field('type') == "box") {
            ?>
            <div class="grid-item grid-item--<?php echo get_field('type');?> <?php echo get_field('position');?>" data-tooltitle="Hood Baroque" data-tooltip="Design | 3D modeling | Drawings">
                <a class="linkpost" href="<?php echo get_post_permalink()?>"><img src="<?php the_field('picture');?>" alt=""></a>
                <a href="<?php echo get_post_permalink()?>" class="text">
                    <img src="<?php the_field('svg_picture');?>" alt="">
                </a>
            </div>
    <?php

        } else {
    ?>
    <div class="grid-item grid-item--<?php echo get_field('type');?> left" data-tooltitle="Hood Baroque" data-tooltip="Design | 3D modeling | Drawings">
        <a class="linkpost" href="<?php echo get_post_permalink()?>"><img src="<?php echo get_field('picture');?>" alt=""></a>
            <a href="<?php echo get_post_permalink()?>" class="text">
                <img src="<?php echo get_field('svg_picture');?>" alt="">
            </a>
    </div>

    <?php 
        }

     endwhile;
    wp_reset_postdata();  
endif;

die();
}

How i can realize search by tags ONLY?