HTML Code

<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=UTF-8>
  <meta name=viewport content="width=device-width,initial-scale=1">
  <title>form Attribute — input outside</title>
  <style>body{font-family:system-ui,sans-serif;margin:0;padding:1.5rem;background:#f8fafc;color:#334155;line-height:1.7}h1{margin:0 0 1rem;font-size:1.25rem;color:#0f172a}.layout{display:flex;flex-wrap:wrap;gap:1rem;max-width:560px}aside,form{flex:1 1 220px;padding:1.25rem;background:#fff;border:1px solid #e2e8f0;border-radius:8px}aside{background:#f1f5f9}label{display:block;margin-bottom:.35rem;font-size:.875rem;font-weight:500;color:#475569}input[type=text]{width:100%;box-sizing:border-box;padding:.45rem .6rem;border:1px solid #cbd5e1;border-radius:6px;font:inherit}.hint{margin-top:1rem;font-size:.8125rem;color:#64748b;max-width:560px}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>
</head>
<body>
  <h1>form — coupon in sidebar</h1>
  <div class=layout>
    <form id=orderForm action=/checkout method=post onsubmit=return!1>
      <p style=margin:0;font-size:.875rem>Main checkout form (hidden cart id only).</p>
      <input type=hidden name=cartId value=12345>
    </form>

    <aside>
      <label for=coupon>Coupon code:</label>
      <input id=coupon name=coupon form=orderForm placeholder=SAVE10>
    </aside>
  </div>

  <p class=hint>The coupon input lives in <code>&lt;aside&gt;</code> but submits with <code>orderForm</code> via <code>form="orderForm"</code>.</p>
</body>
</html>

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HTML form Attribute — Input Outside the Form