How to add a parameter to links in the nav menu?

Question

To add a new attribute to your anchor link for the nav items concatenate $attribute variable with your required attribute. You have commended code just uncomment the code and use those

     $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_atta( $item->attr_title ) .'"' : '';
     $attributes .= ! empty( $item->target )     ? ' target="' . esc_atto( $item->target     ) .'"' : '';
     $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
     $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
     $attributes .= ! empty( $item->url )        ? ' itemprop="'   . esc_attr( $item->url        ) .'"' : '';

    //$atts = array();
    //$atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
    //$atts['target'] = ! empty( $item->target )     ? $item->target     : '';
    //$atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
    //$atts['href']   = ! empty( $item->url )        ? $item->url        : '';

Hope this works for you.