Hi,
When outputting the field value, you can add the attribute to the HTML code. For example:
<a href="<?php echo rwmb_meta( 'field_URL_id' ); ?>" target="_blank"><?php echo rwmb_meta( 'field_URL_id' ); ?></a>
to open the URL in a new tab. Read more on the documentation https://docs.metabox.io/displaying-fields-with-code/