You don’t! The free license for HitFilm Express includes the full editing and 3D compositing toolset, over 400 effects and presets, and all exportion options. The free license is fully functional, and has no time limit, so you can use the free software forever, if you wish. If you want even more features, you can optionally purchase add-on packs to expand the software’s functionality.
Alternatively, HitFilm Pro includes all add-on pack features, plus exclusive effects which are not available for HitFilm Express, and features such as OpenFX support for using third party plugins.