@api.route('/listings', methods=['POST'])
@jwt_required
def create_listing():
    payload = request.json
    listing = listing_svc.create(payload)
    return listing


def create(payload):
    listing = ListingSchema().load(payload, db.session).data


class ListingSchema(ModelSchema):
    id = field_for(Project, 'id', dump_only=True)
    creator_user_id = field_for(Project, 'creator_user_id')
    # ...

    @pre_load
    def set_creator_id(self, data):
        current_user = flask_jwt_extended.get_current_user()
        data['creator_user_id'] = current_user.id

with client.application.app_context():
    rv = client.post('/listings',
        # ...
    )

fake_payload = {}
with client.application.test_request_context('/listings', headers={'Authorization': 'Bearer ' + access_token}):
    create(fake_payload)