Hi!

You can add this attribute with the snippet below:
(Add it to functions.php in your child theme)

    function poka_link_onclick( $linkID , $link_type = 'desktop' ) {

        $output = apply_filters('poka_add_attr_aff_link', "");
        if( get_field('affiliate_onclick_mobile',$linkID) && $link_type === 'mobile' ) {
            $output .= "onclick='".get_field("affiliate_onclick_mobile",$linkID)."'";
        }
        else if( get_field('affiliate_onclick',$linkID) ) {
            $output .= "onclick='".get_field("affiliate_onclick",$linkID)."'";
        }

        $output .= ' rel=”sponsored”';

        return $output;
    }