style.css

.container{
  text-align: center;
  margin-top: 260px;
}
.btn{
  border:2px solid #3498db;
  background: none;
  padding: 10px 20px;
  font-size: 20px;
  font-family: "montserrat";
  cursor: pointer;
  margin: 10px;
  transition: 0.8s;
  position: relative;
  overflow: hidden;
}
.btn1,.btn2{
  color: #3498db;
}

.btn1:hover,.btn2:hover{
  color: #fff;
}
.btn::before{
  content: "";
  position: absolute;
  left: 0;
  width: 100%;
  height: 0%;
  background: #3498db;
  z-index: -1;
  transition: 0.8s;
}
.btn1::before{
  top: 0;
  border-radius: 0 0 50% 50%;
}
.btn2::before{
  bottom: 0;
  border-radius: 50% 50% 0 0;
}
.btn1:hover::before,.btn2:hover::before{
  height: 180%;
}