You just add it to your original footage, and accept whatever quality degradation that grain inherently provides.

Any movie or TV show is ultimately going to be streamed in lots of different formats. And when grain is added, it's often on a per-shot basis, not uniformly. E.g. flashback scenes will have more grain. Or darker scenes will have more grain added to emulate film.

Trying to tie it to the particular codec would be a crazy headache. For a solo project it could be doable but I can't ever imagine a streamer building a source material pipeline that would handle that.