You add far-newspaper to the setting.
Some hints: Discourse currently uses FontAwesome 6, so when you search for icons, make sure that you select the correct version, otherwise the names might be different.
When you look at the icon Newspaper Classic Regular Icon | Font Awesome, you can tell whether it’s a regular, solid, or brand icon by looking at the HTML example.
This one says “fa-regular”, so you know it’s a regular icon, and combined with the description of the site setting “Use no prefix for solid icons, ‘far-’ for regular icons, and ‘fab-’ for brand icons,” you know to use far- as the prefix.