Woocommerce dropdown in the checkout

Hi there hoping someone can help here as i’ve created a drop down box in the checkout all seems to work ok however i want it to be a required field i know i can use the
required = true line but as it already has an option selected as it just uses the first option on the list i have tried using a blank field but no joy

here is my code so far as i’m sure other would want to use this

echo '<div id="my_custom_checkout_field"><h3>'.__('How you found us').'</h3>';

    woocommerce_form_field ( 'my_field_name', array(

	    'type'          => 'select',
		'class'         => array('my-field-class form-row-wide'),
        'label'         => __('Choose an option'),
        'placeholder'   => _x('Phone', 'placeholder', 'woocommerce'),

		'options'     => array(
	 	'option_a' => __( '','' ),
        'option_b' => __('Google', 'Google' ),
        'option_c' => __('Medical referral', 'Medical_referral' ),
		 'option_d' => __('Snorelab mobile app', 'Snorelab_mobile_app' ),
        'option_e' => __('Bing', 'Bing' ),
		'option_f' => __('Yahoo', 'Yahoo' ),
        'option_g' => __('Leaflet', 'Leaflet' ),
		 'option_h' => __('Newsletter', 'Newsletter' ),
        'option_i' => __('Other', 'Other' ),
        ),
		'required'    => true,

        ), $checkout->get_value( 'my_field_name' ));