To assign a manager to a new order go to order card. In 'Manager' field click on 'Not selected', choose a needed manager in a dropdown list and save changes.
You can also press 'Actions' - 'Assign manager':
You can change a manager or delete him in order card by editing a value of this field:
Also, a manager is able to assign an order to him. He needs to press “Actions” - “Assign order to me” in order card:
It is possible to assign a manager via bulk action for a whole segment by orders: