def METHOD_NAME(self, username, password, body='', scope=None,
                         include_client_id=None, **kwargs):
    """Add the resource owner password and username to the request body.
    The client makes a request to the token endpoint by adding the
    following parameters using the "application/x-www-form-urlencoded"
    format per `Appendix B`_ in the HTTP request entity-body:
    :param username:    The resource owner username.
    :param password:    The resource owner password.
    :param body: Existing request body (URL encoded string) to embed parameters
                 into. This may contain extra paramters. Default ''.
    :param scope:   The scope of the access request as described by
                    `Section 3.3`_.
    :param include_client_id: `True` to send the `client_id` in the body of
                              the upstream request. Default `None`. This is
                              required if the client is not authenticating
                              with the authorization server as described
                              in `Section 3.2.1`_.
    :type include_client_id: Boolean
    :param kwargs:  Extra credentials to include in the token request.
    If the client type is confidential or the client was issued client
    credentials (or assigned other authentication requirements), the
    client MUST authenticate with the authorization server as described
    in `Section 3.2.1`_.
    The prepared body will include all provided credentials as well as
    the ``grant_type`` parameter set to ``password``::
        >>> from oauthlib.oauth2 import LegacyApplicationClient
        >>> client = LegacyApplicationClient('your_id')
        >>> client.prepare_request_body(username='foo', password='bar', scope=['hello', 'world'])
        'grant_type=password&username=foo&scope=hello+world&password=bar'
    .. _`Appendix B`: https://tools.ietf.org/html/rfc6749#appendix-B
    .. _`Section 3.3`: https://tools.ietf.org/html/rfc6749#section-3.3
    .. _`Section 3.2.1`: https://tools.ietf.org/html/rfc6749#section-3.2.1
    """
    kwargs['client_id'] = self.client_id
    kwargs['include_client_id'] = include_client_id
    return prepare_token_request(self.grant_type, body=body, username=username,
                                 password=password, scope=scope, **kwargs)