WordPress query in which condition uses custom field

You’re already on the right track, you just need to add the meta key and meta value args to the query

$args = array(
    'post_type'      => 'interviews', 
    'posts_per_page' => 6, 
    'post_status'    => 'publish', 
    'paged'          => $paged
    'meta_key'       => 'speakers_0_speakers_relation',
    'meta_value'     => '76'
);
$loop = new WP_Query($args);