my mistake. This code is ok
function vv_cpt_acf_permalink($post_link, $post = 0)
{
if ($post->post_type === 'team') {
$field_value = get_field("datestart",$datestart);
return $post_link . '/?datestart='.$field_value);
} else {
return $post_link;
}
}
add_filter('post_type_link', 'vv_cpt_acf_permalink', 1, 3);
but… how to always add this parameter, even if someone wants to remove it from the browser bar?