Hi Marco! Take a look at this and tell me if it works
<?php
$define_testimonials = get_field('define_testimonials');
$args = array(
'post_type' => 'testimonial',
'posts_per_page' => 12,
'orderby' => 'menu_order',
'order' => 'ASC',
);
if($define_testimonials) {
// I understand the field returns the ids separated by comma so we convert the string to an array
$array_ids_testimonials = explode(',', $define_testimonials);
// pass the IDs array into post__in arg
$args['post__in'] = $array_ids_testimonials;
}
$loop = new WP_Query($args);