<div id="btn" onclick="showWindow()">Вход</div>
<div id="content_window"></div>

#btn{
 position:absolute;
 top: 50vh;
 left: 50vw;
 z-index:1;
}
#content_window{
 position:absolute;
 height:100vh;
 width:100vw;
 z-index:10;
 background:lightgrey;
 display:none;
}

function showWindow(){
  var el = document.getElementById("content_window");
  el.style.display = 'block';
}