To place a reservation in a box, you must first create a reservation. For more information on how to create a reservation, see How do I create a reservation? of How do I create a reservation for the Virtual Vending Machine?
A reservation can be placed in a box at "Overview", "Reservations" or at "Lockers". For more information about the other pages, see:

You will see an overview of all your reservations in the queue. Click on the reservation you wish te place in a box.
The details of the reservation are shown at the right.
At the top of a reservation you can see whether it has already been assigned to a box. Click on "Reserve box" at the top right.

A popup will appear with a form to choose your box. The next available box is suggested by default. You can also select multiple boxes.
If you choose multiple boxes, the customer will only receive 1 code to open the different boxes. When entering the code, the different boxes open simultaneously.
You can switch between lockers via the dropdown menu.
Click on "Reserve box" to assign a box.

The customer will automatically receive his code x hours before the pick up hour. You can determine this time interval yourself in the settings.