Font Awesome 4 Icons: Web Application Icons
fa fa-external-link
How to Use on a button:
Code Example
<!DOCTYPE html>
<html>
<head>
<title>Font Awesome Icons Demo</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
</head>
<body>
<h1>fa fa-external-link</h1>
<i class="fa fa-external-link"></i>
<i class="fa fa-external-link" style="font-size:20px"></i>
<i class="fa fa-external-link" style="font-size:30px"></i>
<i class="fa fa-external-link" style="font-size:40px;color:brown"></i>
<i class="fa fa-external-link" style="font-size:60px;color:brown"></i>
<i class="fa fa-external-link" style="font-size:80px;color:brown"></i>
<br>
<p>How to Use on a button:</p>
<button style="font-size:20px">Button <i class="fa fa-external-link"></i></button>
</body>
</html>