JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menuSelected"><span></span></div>

CSS

#menuSelected {
    position: relative;
    width: 250px;
    height: 35px;
    background-color: #368EC5;
}
span {
    border-top: 10px solid #368EC5;
    border-right: 10px solid transparent;
    border-left: 10px solid transparent;
    position: absolute;
    bottom: -10px;
    left: 20px;
}