It is not possible to insert a bookmark inside the textbox in order to create a cross reference. Nor there is a possibility to insert a reference name in its editing box (pencil icon).
Example for the use of this feature: in a book where all the biographical notes are in small boxes throughout the pages, I inserted them as pdf images in the 3.4.5 version. In this way I can give references in other pages, and compose the list in the table of contents with page numbers.
This feature missing, I'm obliged to return to the 3.4.5 version.