<a 
  href="https://app.memberstack.com/signup/YOUR_PLAN_ID" 
  target="_blank" 
  rel="noopener noreferrer"
  style="
    display: inline-block;
    padding: 12px 24px;
    background-color: #ff007a;
    color: white;
    font-weight: bold;
    border-radius: 8px;
    text-decoration: none;
    cursor: pointer;
  "
>
  Subscribe
</a>