Change WC Coupon Placeholder

wccptxt.php

function my_coupon_strings( $translated_text, $text, $domain ) {
    switch ( $translated_text ) {
        case 'Coupon code' :
            $translated_text = __( 'My New Coupon Code Text', 'woocommerce' ); // Change text here
            break;
    }
    return $translated_text;
}
add_filter( 'gettext', 'my_coupon_strings', 10, 3 );