add multiple values (array) to post meta_input

You can simply store the meta as an array… Also to hell with add_post_meta… I never use it. update defaults to add if the meta doesn’t exist for the post.

update_post_meta($post_id, "_event_musikstil", $event_musikstil);

wordpress will store the value as a serialized array then so just retrieve it like

get_post_meta($post_id, "_event_musikstil", TRUE);

Hope this helps.

Another way, if you don’t want to store a serialized array is to use explode and implode

        $metaStr = get_post_meta($post_id, 'your_key', true);
        if(!empty($metaStr)){
            $meta_arr = explode(',', $metaStr); // assuming you stored as , separated
        }else{
            $meta_arr = []; // empty array
        }
        foreach($newValuesToAdd as $newValueToAdd) {
            $meta_arr[] = $newValueToAdd;
        }
       update_post_meta($post_id, 'your_key', implode(',',$meta_arr));