.container {
  position: relative;
  width: 400px;
  height: 150px;
  background: red;
}

.container:after {
  position: absolute;
  content: '';
  width: 150px;
  height: 75px;
  background: #fff;
  left: 125px;
  border-bottom-right-radius: 90px;
  border-bottom-left-radius: 90px;
}