<a href="#" class="btn">Hover me!</a>

body {
  background-color: #333;
  padding: 50px;
  font-family: sans-serif;
}

.btn {
  display: block;
  border: 1px solid #FFFFFF;
  background-color: transparent;
  max-width: 250px;
  text-align: center;
  padding: 10px;
  color: #FFF;
  text-decoration: none;
  border-radius: 10px;
  overflow: hidden;
  position: relative;
  transition: all 0.5s;
}

.btn:before {
  content: '';
  display: block;
  background: linear-gradient(66.7deg, #EE3E7C 9.43%, #01B37D 92.62%);
  width: 100%;
  height: 100%;
  position: absolute;
  left: 0;
  top: 0;
  z-index: -1;
  opacity: 0;
  transition: all 0.5s;
}

.btn:hover:before {
  opacity: 1;
}

.btn:hover {
  border: 1px solid transparent;
}