Add class to functions.php custom code to call font awesome icon

There’s no need to add another class. The code you’re using already creates separate classes and ID. You could do this with just css:

Inspect your site with console, but your div label should be technical_documents so your code would be:

#tab-title-technical_documents a::before {
  content: "\f02d";
  font-family: 'Font Awesome 6';
  font-weight: 900;
}

You didn’t mention which version of font-awesome you’re using, so you may have to change the font-family to match your version. (Again, you could check your console and see which version the other two tabs are using.)