Writer, subscribed x in fn=F3

Hello!
Please, can someone give me advice how to write subscibed x in writer, fn+F3?
I tried with right click-formatx-csubx but I just get it under x, not realy subscribed.

Please, what to choose?
I have version 4,0

Many thanks!

https://wiki.documentfoundation.org/Documentation/Publications#LibreOffice_Writer_Guide