<form method="post" name="contactform" action="#">
<input type="text" class="pop" name="name" placeholder="your name">
<br><br>
<input type="text" class="pop" name="email" placeholder="your email address">
<br><br>
<textarea class="notes6" name="message" placeholder="your message"></textarea>
<br><br>
<input type="submit" value="submit" class="sub">
</form>

.pop {background-color: #18191b;
width:200px; height:30px;
color:#ffffff;
font-size : 14px; 
border: 2px solid #272626;
border-radius:4px;}
 .notes6
        {
           background-color: #18191b;
            border: 1px solid #000;
            border-radius: 8px;
            box-shadow: inset 0 1px 2px rgba(0, 0, 0, 0.1);
            line-height: 31px;
            font-family: Tahoma;
 font-size: 20px;
            padding: 0px;
            width:400px;
height:200px;

color:#fff;
  border-radius:10px;
 border: 2px solid #272626;
overflow: hidden;

}
.sub {
background: #777;
color: white;
border: 1px solid #000;
height: 30px;
width: 70px;
font: normal 15px arial,sans-serif;
text-shadow: none;
cursor:pointer;
}

.sub:hover {
background: #555;
color: white;
border: 1px solid #000;
height: 30px;
width: 70px;
font: normal 15px arial,sans-serif;
text-shadow: none;
cursor:pointer;
}

<iframe src="https://www.netbax.net/tab/css/css-forms-property.html" style="width: 550px; height:550px;" frameborder="0"></iframe>