If else statement. Basics.

<!DOCTYPE html>
<html lang=en>
<head>
<title></title>
<meta charset=utf-8>
<meta name="viewport" content="width=device-width">
<style>

#div_1{
  background-color:red;
  display:inline-block;
  padding:5px;
  position:absolute;
}

#inp_1{
  font-size:25px;
}

#bt_1{
  font-size:25px;
}

</style>
</head>
<body>

<div id='div_1'>
<input id='inp_1' type='search'  placeholder='type'/>
<button id='bt_1'>Check</button>
</div>


<script>
var x = 'letmein';

bt_1.onclick=function(){
if(inp_1.value==''){
   inp_1.value='Fill in please';
   }else if(inp_1.value == x){
     alert('welcome');
      location.href="https://www.youtube.com/embed/8nMVrN67zm4";
  }else{
        alert('wrong password');
  }
}

</script>
</body>
</html>