HTML

<button id="s6" href="#">hi</button>

CSS

#s6
{
border-radius: 10px;
transition: 1s ease-in-out;
background: linear-gradient(white,rgb(255, 255, 255));
background-position: center;
box-shadow: 0px 0px 10px 5px inset rgba(0, 0, 0, 0.671);
}
#s6:hover
{
color: white;
border-radius: 50px;
transition: 1s ease-in-out;
box-shadow: 0px 0px 10px 100px inset rgb(0, 0, 0);
}