As of this moment I don’t think u can watch it on any official streaming services, which means u will have to use an “illegal site” if it’s even available to watch.
Also just an FYI but movies in particular can take quite a while to be added to streaming services. For example following its release in Japan, Weathering With You took like around a year, so it might not even be available to watch right now.
But if u want anymore info on it u should ask at the Given Wiki, as people over there will probably know more:
https://given.fandom.com/f