JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.header {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    background: #000;
    height: 110px;
}

.header:after {
    content: '';
    position: relative;
    top: 100%;
    display: block;
    margin: 0 auto;
    background: red;
    width: 50px;
    height: 25px;
    border-radius: 0 0 50px 50px;
}