add_action( 'init', function() {
    remove_post_type_support( 'post', 'excerpt' );
} );
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    $meta_boxes[] = [
        'title'      => 'Test',
        'post_types' => 'post',
        'fields'     => [
            [
                'type' => 'textarea',
                'id'   => 'excerpt', // This is the must!
                'name' => 'Fake content',
            ],
        ],
    ];

    return $meta_boxes;
} );

add_filter( 'rwmb_excerpt_field_meta', function() {
    $post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );
    return get_post_field( 'post_excerpt', $post_id );
} );
add_filter( 'rwmb_excerpt_value', '__return_empty_string' );