#1. Add this code to the Code Injection >Header or Code Block

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.1.1/css/all.min.css" integrity="sha512-KfkfwYDsLkIlwQp6LFnl8zNdLGxu9YAA1QvwINks4PhcElQSvqcyVLLD9aMhXd13uQjoXtEKNosOWaZqXgel0g==" crossorigin="anonymous" referrerpolicy="no-referrer" />

#2. Add this code to the Custom CSS box and Save

a.btn:before {
    content: "\f095";
    font-family: "Font Awesome 6 Free";
    font-weight: bold;
}

#3. Result