Create a css class to allow icon to be recognized
font-family: FontAwesome, "Open Sans", Verdana, sans-serif;
and then set the created class to the item
Do not use the "Value Placeholder" to include the icon, if you do this, the code will be transformed into text. Search for the icon unicode at this link and then set it to the custom attributes field as the picture below.
and then refresh the page.
Oracle APEX 5.0 comes with FontAwesome version 4.2.0 on Universal Theme. If you want to upgrade for the last version to add some icons do the steps as follows