default values from shortcode in checkbox

https://contactform7.com/getting-default-values-from-shortcode-attributes/

I follow this, and create a custom php code, work for text inputs, but dosan’t work in checkboxs,

I would like to control the checkboxes that are checked by default via PHP, using this it worked for only 1 checkbox field, I need it to work for one more option, I tried using array , but it didn’t work.
my code that works for just 1 check

In form: [checkbox checkbox1 default:shortcode_attr use_label_element “option a” “option b” “option c”]

In PHP:

    add_filter( ‘shortcode_atts_wpcf7’, ‘custom_shortcode_atts_wpcf7_filter’, 10, 3 );

    function custom_shortcode_atts_wpcf7_filter( $out, $pairs, $atts ) {

        $out[“checkbox1”] = “option b”;

        return $out;

    }