Metabox value for post loop?

Thanks guys.I did it with WP_Meta_Query (https://codex.wordpress.org/Class_Reference/WP_Meta_Query)

Solution:

<?php $query = new WP_Query( array( 
             'meta_key' => 'j4cob_yaz_ayarlar_show_on_slider_',
             'meta_value'   => 'show-on-slider',
             'posts_per_page' => 6
             ) ); ?>
   <?php if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post(); ?>
.
Loop
.
<?php endwhile; 
             wp_reset_postdata();
             else : ?>
            <p><?php _e( 'Sorry, no posts matched your criteria.' ); ?></p>
          <?php endif; ?>