[TYPO3-ttnews] How to set tt_news-field datetime to readonly for BE-usergroup

[Tobias Schaefer]

Hi,

I'd like to set the tt_news-field datetime to readonly for a certain BE-usergroup. 
I tried this typoscript in the usergroup-ts-config but it didn't worked:
TCEFORM.tt_news.datetime.config.readOnly = 1

I've TYPO3 v4.2.8 and tt_news v2.5.2 installed.

Any suggestions?

Cheers,
Tobias