token-generator | A library to generate random strings

You can add custom token issuers in the following way in 5.10.0 instead of token_generator.

You will need to make sure your Firebase Admin sdk is initiated and running before the function proceeds

It's not possible to use the Firebase Functions SDK to invoke onRequest type functions. The Firebase SDK implements the client side of a callable function that you declare with onCall. You're using onRequest here, which means you're writing a standard HTTP type function. For that type of function, you should use a standard HTTP client (not the Firebase SDK). If you actually did want to use the Firebase SDK to invoke your function, you will have to write a callable function instead. Note that callable functions have their own spec, and you won't be able to easily invoke them from postman.

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I believe that you have a mistake on this line:

If you have syntax problems like this, the docs are your friend. From the reference documentation for Auth.createCustomToken:

createCustomToken(uid: string, developerClaims?: Object): Promise

...

Parameters

uid: string The uid to use as the custom token's subject.

Optional developerClaims: Object

Optional additional claims to include in the custom token's payload.

So the second argument to createCustomToken needs to be an object with the custom claims you want to add. My guess is that the boolean in the question you linked was a valid call in an older version of the SDK.

From my understating of Firebase Custom Token Auth, you cannot send a plain unencrypted token like that. In order for that to work, you would have to generate the token based on a uid, which could be the 123456 by using the same code you were already have: