Right-click on the Fonts folder in the project bar and import a font of your choosing. WOFF seems to be the preferred type, but True Type (ttf) will work as well. Here is a link to a WOFF-Converter.
My project uses Bahnschrift and Font Awesome 6.
You will have to rewire all text objects by selecting the imported font.
Now when you export, C3 should include the font in the package.