JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    margin: 30px;
    border-radius: 10px;
    border: 1px solid #ccc;
    position: relative;
    min-height: 150px;
}
div::before {
    content: '';
    border: solid #fff;
    border-width: 20px 0 0;
    border-radius: 35px 0 0 0;
    width: 35px;
    height: 15px;
    position: absolute;
    top: 20px;
    left: -25px;
    box-shadow: -1px -1px 0 0 #ccc, inset 1px 1px 0 0 #ccc;
    clip: rect(-1px 25px 35px -1px);
}