Sign in With Custom Token

Simple questions:

1. Is it possible to perform a firebase authentication using a custom token generated from a backend service? According to the official documentation, YES, but unfortunately the REALITY is different. Once you try to perform this code
```
const firebase = require('firebase');

const token = '';

const app = firebase.initializeApp({
databaseURL: "https://my-fancy-instance.firebaseio.com",
});

app.auth().signInWithCustomToken(token);
```

Then you get an error for an invalid API KEY, which leads to my 2nd question:

2. Why in the name of the holy motherboard we have a custom token authentication if we ALSO MUST set the **secret**, which is the API KEY?