I was searching for the answer to this too... and I managed to figure it out:
In the Story Editor menu, click on "Insert" and then "Glyph"... the Character Palette window will open. Click on the leftmost option: Show/Hide Enhanced Palette and the Enhanced Character Palette window will open. In the font selection option, choose "Boxicons - Regular" and the full character set for the boxicons font will appear. Choose the boxicon icon you would like to insert in your text by double clicking on it and it will appear in the Glyphs to insert panel. Then just click on "Insert" and the icon will be inserted into the Story Editor text. Don't worry if it appears as a set of horizontal lines in Story Editor... in your actual document, the icon will be fully displayed.