<span class="newsletter-offer-text-promocode">Текст в буфер</span>

  let newsletterPromoCode = document.querySelector(".newsletter-offer-text-promocode");

    if(newsletterPromoCode !== null) {
      newsletterPromoCode.onclick = function() {
        document.execCommand("copy");
      }
    newsletterPromoCode.addEventListener("copy", function(event) {
      event.preventDefault();
      if (event.clipboardData) {
        event.clipboardData.setData("text/plain", newsletterPromoCode.textContent);
        
        newsletterPromoCode.textContent = "Copied!";
        var oneSecond = 2000; 
        setTimeout(function() {
         newsletterPromoCode.textContent = "discount10";
      }, oneSecond);
      }
    });
    }