css skew element and get inner rounded border top

.header {
  border-top: 20px solid blue;
  height:100px;
  position: relative;
  overflow: hidden;
}
.header:before,
.header:after {
  content: "";
  vertical-align:top;
  display: inline-block;
  transform-origin: top right;
  transform: skew(-40deg);
}

.header:before {
  height: 100%;
  width: 50%;
  border-radius: 0 0 20px 0;
  background: blue;
}

.header:after {
  height: 20px;
  width: 20px;
  margin-left:-1px;
  background: radial-gradient(circle at bottom right, transparent 68%, blue 73%);
}


/*to illustrate different values of skew*/
.header:before,
.header:after {
  animation:change 2s linear infinite alternate;
}

@keyframes change{
  from{transform: skew(0deg);}
  top{transform: skew(-40deg);}
}

<div class="header"></div>