<a href="#" class="cp_btn">button</a>

a.cp_btn {
  position: relative;
  display: block;
  width: 160px;
  padding: 0.8em;
  text-align: center;
  font-weight: bold;
  text-decoration: none;
  color: #4CAF50;
}
a.cp_btn:before {
  position: absolute;
  content: '';
  width: 100%;
  height: 4px;
  top:100%;
  left: 0;
  border-radius: 3px;
  background:#4CAF50;
  transition: .4s;
}
a.cp_btn:hover:before {
  top: calc(100% - 3px);
}