<form method="POST" action="odeme">
  <div class="bidding__center center">
    <div class="bidding__wrapper">
      <div class="bidding__item js-bidding-item" style="display: block;">
        <div class="payment">
          <div class="payment__fieldset">
            <div class="payment__row">
              <div class="field">
                <div class="field__label">Ad</div>
                <div class="field__wrap">
                  <input class="field__input" type="text" name="ad" placeholder="Ad" required="">
                </div>
              </div>
              <div class="setting__row" style="margin-top: 29px;">
                <div class="field">
                 <div class="price__stage">Kupon Kodu Ekle</div>
                 <form class="subscription" method="POST">
                  <input class="subscription__input" type="text" name="kod" placeholder="Kupon Kodu Ekle">
                  <div class="payment__btns">
                    <button type="submit" name="kuponbak" class="button payment__button">Uygula</button>
                  </div>
                </form>
              </div>
            </div>
            <div class="payment__btns">
              <button type="button" class="button payment__button js-bidding-button">İleri</button>
            </div>
          </div>
        </div>
      </div>
      <div class="bidding__item js-bidding-item" >
        <!-- Form 2. Alan -->
      </div>
      <div class="bidding__item js-bidding-item">
        <!-- Form 3. Alan -->                       
      </div>
    </div>
  </div>
</form>