[Typo3] mailformplus: setting subject via URL

[Namuk Tscholak]

Hi,

I want to set the subject of the mailformplus-form via URL.

http://www.dummysite.com/mailform.html?subject=MyOwn%20Subject
(simulateStaticDocuments used!)

To set the subject static works perfect like this:
Setup:
plugin.tx_thmailformplus_pi1.default.email_subject = MyOwn Subject

To use the GPvar don't work. :`-(
Setup:
plugin.tx_thmailformplus_pi1.default.email_subject = GPVar:subject

How can I do that?
Is there the simulateStaticDocument-function the problem or the PHP 
feature register_globals=off?

regards
namtscho