How to read and write the custom attributes of a Word 2016 document using Word JavaScript API 1.1?

Question

We are developing an Add-In for Word 2016 using the Word JavaScript API 1.1 libraries, that are supported by Word 2016.

The goal of this Add-In would be to read and write the custom attributes of a Word document.

However, we find no documentation explaining how to achieve this goal.

All the examples we found refer to the "context.document.properties.customProperties" that does not exist in the Word JavaScript API 1.1; it seems to be part of the Word JavaScript API 1.3, which however is not supported on Word 2016 (only on Word 2019 or 365).

So, our question is: how to read and write the custom attributes of a Word 2016 document using Word JavaScript API 1.1?