SOURCE

<div class='container'>
  <div class="c_left"></div>
  <div class="c_right"></div>
</div>

.container{
  width:800px;
  height:400px;
  background:#708cfe;
  margin:30px auto;
  border:1px solid #000;
  border-radius:20px;
}
.c_left{width:35%;height:100%;background:#fff;border-radius:20px 0 0 20px;}