To the people asking for the activation key, you just have to add the game to the library and download it from here. This is not going to give you a key to add it to another platform like Steam.
If you want, on Steam or Heroic you can then add it to show up on that interface. Example: I have Genshin Impact on Epic and use Heroic launcher to download and update it, but since sometimes I want to use Steam Link to play on my TV, I added it to Steam. It is not managed by Steam, but it shows up on Steam's interface.