[TYPO3-english] Customizing flexforms.xml ( TYPO3 6.2.5 and News (latest version)

[Bert]

Hello,

I am customizing the flexform.xml for the News-plugin since I would like 
restrict the rendered categories to a certain pid.

I can do that by using the marker ###STORAGE_PID### in the flexform:

                 <!-- Category -->
                     <settings.categories>
...
                                 <foreign_table_where> AND 
(sys_category.pid = ###STORAGE_PID###) AND 
(sys_category.sys_language_uid = 0 OR sys_category.l10n_parent = 0) 
ORDER BY sys_category.sorting</foreign_table_where>

...

                     </settings.categories>

The above works but how can I get the startingPoint (folder where the 
news items reside) as a marker into the flexform?

Tried several variants from this example but could not get this working:
http://www.typo3lexikon.de/typo3-tutorials/tipps-und-tricks/marker-in-flexforms.html

Any suggestions?

Thanks in advance!

Regards,
Bert