<div class="wrapper">
  <input type="text">
  <div class="button">
    <a href="#">Btn</a>
  </div>
</div>

.wrapper {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 10px;
}

input {
  padding: 10px;
}

.button {
  width: 100%;
  max-width: 100px;
}

.button a {
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 10px;
  color: black;
  text-transform: uppercase;
  text-decoration: none;
  border-radius: 5px;
  border: 1px solid black;
  transition: background .5s ease, color .5s ease;
}

.button a:hover {
  color: white;
  background: #000;
}