在php wordpress插件开发中选择一个默认值

I have dropdown which generates list of countries in wordpress plugin.

By default it selects first country, i want to change it to poland

What should i do?

if( !get_user_meta( get_current_user_id(), 'user_nationality', true ) ) {
    $countries_obj = new WC_Countries();
    $countries = $countries_obj->__get('countries');
    $fields['billing']['user_nationality'] = array(
        'type'      => 'select',
        'label'     => __( 'Nationality', 'mangopay' ),
        'options'   => $countries,
        'required'  => true,

    );
}

it' my own woocomerce related plugin, i am a beginner.

i followed this as well https://codex.wordpress.org/Creating_Options_Pages.

Use "std" => 'default_value', after 'required' => true,