How do I add a font awesome icon

I have this code and would like to prepend it with a FA Icon that is part of the mailto link

<?php
$theEmail = get_sub_field('staff_email_address');
encryptx($theEmail, array( 'css_class' =>
'', 'css_id' =>'', 'echo' => 1 ));
?>

https://wordpress.org/plugins/cryptx/