ibooks author link word to glossary

I already know how to link a word which is on a regular (or blank) Page to the glossary.

How does it work for words which are typed into a scroll-box/scroll-field (->See picture below)?

I can't choose the field "add link" ("link hinzufügen" in german) which is just below the information- and Media- Symbols to link the selected word to the glossary.

Wenn need advice :)