Event.belongsToMany(models.Employee, { as: 'Employee', through: 'EventEmployee' });

 async test(req, res) {
        try {
            console.log(req.body);
            await Employee.create({ name: 'nice owner', event: [{ name: 'nice property' }, { name: 'ugly property' }] }, { include: [Event] });
        } catch (err) {
            res.status(500).send({
                error: 'возникла ошибка при добавлении записи',
            });
            console.log(err);
        }
    },
};

Event.belongsToMany(models.Employee, { as: { singular: "asdasdasd", plural: 'EventEmployees'}, through: 'EventEmployee' });