It’s not possible to secure anything client side, that key is in a plain text in a bundle because the code needs it to use it.
Theoretically, it would be possible to encode it somehow and decode it in runtime, but what would be a point of doing that. It would still be easily accessible if someone is determined to access it. The only proper approach is not to have any secrets in your client app.
Obfuscation/mimization is intended to improve perfomance and hide your source code, so it’s harder to get to the point where you, but there is no way to hide values embedded there.
If you have a key in your app then either it should be designed in a way that leak is not a big problem, or you shouldn’t have that key in your client side code.