Starts a job to fetch the token balance for a given account and token address as of a specific block.

The endpoint returns a URL that can be used to check for the results.

After starting the job:

1. Call the URL that is returned
2. If you get a 425 status code, retry after some time
3. Continue polling until you get the final balance result

Note: Processing will take some time, depending on the how far the requested block is from chain genesis, and depending on the transaction volume of the requested account.