client to send the form field `grant_type` with the value `"password"`, which
    is required in the OAuth2 specification (it seems that for no particular reason),
    while for `OAuth2PasswordRequestForm` `grant_type` is optional.

    Read more about it in the
    [FastAPI docs for Simple OAuth2 with Password and Bearer](https://fastapi.tiangolo.com/tutorial/security/simple-oauth2/).