def METHOD_NAME(self, field_name):
    """
    Raise an appropriate OAuthValidationError error if the field is missing
    """
    field_val = self.cleaned_data.get(field_name)
    if not field_val:
        raise OAuthValidationError(
            {
                "error": "invalid_request",
                "error_description": f"{field_name} is required",
            }
        )
    return field_val