Кастомизация бордера

button { 
  border: 2px solid #000; 
  border-left: none; 
  padding: 15px 30px 15px 60px; 
  border-radius: 10px; 
  background: #fff; 
  position: relative; 
} 
 
button:before { 
  content: ''; 
  position: absolute; 
  top: -2px; 
  left: 20px; 
  width: 20px; 
  height: 2px; 
  background: #000; 
  z-index: 1; 
} 
 
button i { 
  background: #fff; 
  font-size: 50px; 
  color: #ccc; 
  position: absolute; 
  top: 50%; 
  left: -5px; 
  transform: translateY(-50%); 
} 
 
button:hover { 
  border-color: #00f; 
} 
 
button:hover:before { 
  background: #00f; 
}

<link href="https://use.fontawesome.com/releases/v5.2.0/css/all.css" rel="stylesheet" /> 
<button><i class="fas fa-lock"></i>Button</button>