Hi @PowerPat ,
By the HTML field he means the field that was created by code and will be used only as UI element and doesn't directly bind to CDS field.
To make this example work make sure that you followed the article and create a multi-select field (it will not be placed on the form), text field (that will be placed on the form and be hidden by the JavaScript code) and Plugin that will actually populate our multi-select field based on the value of the text field.
Plugins are server side code written in C# that perform some complex business logic. You can find out more about plugins in this official docs and tutorials.
If you have issues with UI part on the portal make sure that you added text field to the form, added JavaScript in the custom JavaScript field in Advance tab on the Entity Form, in the JavaScript you put correct name of your custom field, and after that Sync Configuration (open Portal app and press Sync Configuration on the top right).