How to use the webargs.fields.String function in webargs

def wrapped(*args, **kwargs):
            req_args = webargs_parser.parse({
                'existing_submission_id': webargs_fields.Int(missing=None),
                'cgac_code': webargs_fields.String(missing=None),
                'frec_code': webargs_fields.String(missing=None)
            })
            # Ensure there is either an existing_submission_id, a cgac_code, or a frec_code
            if req_args['existing_submission_id'] is None and req_args['cgac_code'] is None and \
               req_args['frec_code'] is None:
                raise ResponseException('Missing required parameter: cgac_code, frec_code, or existing_submission_id',
                                        StatusCode.CLIENT_ERROR)

            # Use codes based on existing Submission if existing_submission_id is provided, otherwise use CGAC or FREC
            if req_args['existing_submission_id'] is not None:
                check_existing_submission_perms(perm, req_args['existing_submission_id'])
            else:
                # Check permissions for the agency
                if not current_user_can(perm, cgac_code=req_args['cgac_code'], frec_code=req_args['frec_code']):
                    raise ResponseException("User does not have permissions to write to that agency",
                                            StatusCode.PERMISSION_DENIED)
            return fn(*args, **kwargs)

        'comment': fields.String(required=True),
    })
    def _process(self, email, comment):
        self._save_report(email, comment)
        return '', 204

        'cross_type': webargs_fields.String(validate=webargs_validate.OneOf(FILE_TYPE_DICT.keys()))
    })
    def post_submission_report_url(submission, warning, file_type, cross_type):
        return submission_report_url(submission, bool(warning), file_type, cross_type)

}
                  }
        return result


externalapi_args = {
    "input": fields.String(missing=None),
    "apikey": fields.String(missing=None),
    "t": fields.String(missing=None),
    "q": fields.String(missing=None),
    "query": fields.String(missing=None),
    "group": fields.String(missing=None),
    "limit": fields.Integer(missing=100),
    "offset": fields.Integer(missing=0),
    "cat": fields.String(missing=None),
    "o": fields.String(missing="XML"),
    "raw": fields.Integer(missing=0),
    "attrs": fields.String(missing=None),
    "extended": fields.Bool(missing=None),
    "del": fields.String(missing=None),
    "rid": fields.String(missing=None),
    "genre": fields.String(missing=None),
    "imdbid": fields.String(missing=None),
    "tvdbid": fields.String(missing=None),
    "season": fields.String(missing=None),
    "ep": fields.String(missing=None),
    "id": fields.String(missing=None),
    "author": fields.String(missing=None),

    # These aren't actually needed but the way we pass args objects along we need to have them because functions check their value
    "title": fields.String(missing=None),
    "category": fields.String(missing=None),

        'certified': webargs_fields.String(
            required=True,
            validate=webargs_validate.OneOf(('mixed', 'true', 'false'))),
        'sort': webargs_fields.String(missing='modified'),
        'order': webargs_fields.String(missing='desc'),
        'fabs': webargs_fields.Bool(missing=False),
        'filters': webargs_fields.Dict(keys=webargs_fields.String(), missing={})
    })
    def list_submissions(certified, **kwargs):
        """ List submission IDs associated with the current user """
        page = kwargs.get('page')
        limit = kwargs.get('limit')
        sort = kwargs.get('sort')
        order = kwargs.get('order')
        fabs = kwargs.get('fabs')
        filters = kwargs.get('filters')
        return list_submissions_handler(page, limit, certified, sort, order, fabs, filters)

        'filters': webargs_fields.Dict(keys=webargs_fields.String(), missing={})
    })
    def historic_dabs_summary(**kwargs):
        filters = kwargs.get('filters')
        return historic_dabs_warning_summary(filters)

        'file_type': webargs_fields.String(
            required=True,
            validate=webargs_validate.OneOf(FILE_TYPE_DICT.keys() - {'executive_compensation', 'sub_award'})
        ),
        'warning': webargs_fields.Bool(),
        'cross_type': webargs_fields.String(validate=webargs_validate.OneOf(['program_activity', 'award_financial',
                                                                             'award_procurement', 'award']))
    })
    def post_submission_report_url(submission, file_type, **kwargs):
        warning = kwargs.get('warning')
        cross_type = kwargs.get('cross_type')
        return submission_report_url(submission, bool(warning), file_type, cross_type)

        'start': webargs_fields.String(required=True),
        'end': webargs_fields.String(required=True)
    })
    def generate_detached_file(file_type, cgac_code, start, end):
        """ Generate a file from external API, independent from a submission """
        file_manager = FileHandler(request, is_local=is_local, server_path=server_path)
        return file_manager.generate_detached_file(file_type, cgac_code, start, end)

@app.route('/internalapi/getindexerstatuses')
@requires_auth("stats")
def internalapi_getindexerstatuses():
    logger.debug("Get indexer statuses")
    return jsonify({"indexerStatuses": get_indexer_statuses()})


internalapi__getnzbdownloads_args = {
    "page": fields.Integer(missing=0),
    "limit": fields.Integer(missing=100),
    "type": fields.String(missing=None)
}


class SortModelSchema(Schema):
    column = fields.String()
    sortMode = fields.Integer()


class DownloadHistoryFilterSchema(Schema):
    page = fields.Integer(missing=1)
    limit = fields.Integer(missing=500)
    sortModel = fields.Nested(SortModelSchema, missing=None)


@app.route('/internalapi/getnzbdownloads', methods=['POST'])
@requires_auth("stats")
@use_args(DownloadHistoryFilterSchema())
def internalapi_getnzb_downloads(args):
    filterModel = request.json["filterModel"] if request.json and "filterModel" in request.json.keys() else {}
    return jsonify(get_nzb_downloads(page=args["page"], limit=args["limit"], filterModel=filterModel, sortModel=args["sortModel"]))