You can achieve that by modify Modifier of NavigationBar I am attaching example below, You can refer it...

NavigationBar(
    modifier = Modifier
        .shadow(
            elevation = 8.dp,
            shape = RoundedCornerShape(topStart = 20.dp, topEnd = 20.dp)
        )
) {
       // enter code here
}

Here is the output with applied 8.dp shadow you can modify it as your need.