Do you need to query only the items that has `banner_link` value set to `https://www.mypage.com` ? 

If no, you can remove the `meta_query` part from your query.

    <?php
    // function to show home page banner using query of banner post type
    function wptutsplus_home_page_banner() {
    
    // start by setting up the query
    $get_banner = new WP_Query( array(
        'post_type' => 'banners',
        'meta_query' => array(
          array(
            'key' => 'banner_link',
            'value' => 'https://www.mypage.com'
          )
        )
    ));
    
    // now check if the query has posts and if so, output their content in a banner-box div
    if ( $get_banner->have_posts() ) { ?>
            <?php while ( $get_banner->have_posts() ) : $get_banner->the_post(); ?>
    
              <div class="container" align="center"><a href="<?php echo get_post_meta( $post->ID, 'banner_link', true ); ?>"><?php the_post_thumbnail(); ?></a></div>
    
            <?php endwhile; ?>
    <?php }
      wp_reset_postdata();
    }
    ?>

**UPDATE**

As you're using it inside a plugin. You can do like the following: 

    <?php
    // function to show home page banner using query of banner post type
    function wptutsplus_home_page_banner() {
    
    // start by setting up the query
    $get_banner = new WP_Query( array(
        'post_type' => 'banners',
        'meta_query' => array(
          array(
            'key' => 'banner_link',
            'value' => 'https://www.mypage.com'
          )
        )
    ));
    
    // now check if the query has posts and if so, output their content in a banner-box div
    if ( $get_banner->have_posts() ) : while ( $get_banner->have_posts() ) : $get_banner->the_post();
    
      $output = '<div class="container" align="center"><a href="'.get_post_meta( $post->ID, 'banner_link', true ).'">'.get_the_post_thumbnail().'</a></div>';
    
    endwhile;
    endif;
    wp_reset_postdata();
    return $output;
    }
    ?>

And then call it anywhere with `<?php echo wptutsplus_home_page_banner(); ?>` 

Let me know if it works.