<a href"#">予約はこちら</a>

a {
  position: relative;
  display: block;
  width: 200px;
  height: 3em;
  border: 2px solid orange;
  text-align: center;
  padding-block: 2em 0;
  z-index: 1;
}

a::after {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  background-color: orange;
  transition: .5s all;
}

a:hover::after {
  width: 0;
}