[Typo3] Passing the current value to a user function in typoscript

[Robert John de Stigter]

Hi,
 
In a templavoila datastructure I can add typoscript code to an element.
Like this:
 
<TypoScript>
10 = TEXT
10.current = 1
10.case = upper
</TypoScript>
 
This will uppercase the value put in the form. But how can I pass the
current value to a custom function?
This doesn't seem to work.
<TypoScript>
10 = USER
10.current = 1
10.userFunc = myclass->myfunction
</TypoScript>
 
Cheers,
Robert