plus fa fa icon in bootstrap button code example

Example 1: plus sign icon font awesome

Font Awesome 5: <i class="fas fa-plus"></i>

Example 2: plus icon using font awesome

//Add it at in head
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
// Add it where u want to add icons
<i class="fa fa-plus"></i>
// u can also look at font awesome cheatsheet for reference.It will be helpful.