Changing the placeholder text of coupon input field

change-coupon-field-placeholder-text.php

<?php

add_filter( 'gettext_woocommerce', 'ouwoo_coupon_placeholder_text', 10, 3);
function ouwoo_coupon_placeholder_text( $translated_text, $text, $domain ) {
  switch( $translated_text ) {
    case 'Enter coupon code' : 
      $translated_text = "ENTER YOUR TEXT HERE";
      break;
  }
  return $translated_text;
}