Don't put the Account object's ID into the URL. The [?requestid=] from the documentation you mentioned apparently refers to an id related to the request (not the object in question). The API Explorer's URI appears to simply mislead (although I could certainly be missing something here).
In your example, just use this:
https://quickbooks.api.intuit.com/v3/company/0123456789/account
Let the headers and request body do the rest.