I'm sorry for the trouble caused to you.
I believe you are trying to find the "Unique ID" field. Please be informed that, the "Unique ID" field is now moved to the "Widget" section of form builder:
You need to click on the "Widget" tab and input the text "Unique ID" in the search box. add the field to you form from the search result.
Please be informed that the "Hidden Box" field can also be found in the "Widget" section.
Hope this helps.
Do get back to us if you have any questions.