[TYPO3-english] Flexform in custom content element

[Fabian]

Hello,

I've created my own content element and I'm using flexforms to make use 
of some custom fields. How can I make the TYPO3 backend show some of the 
custom fields?

In the attached screenshot you can see the custom element in green, it 
is surrounded by normal content elements in which you can see "header" 
or "bodytext". I'd like to have the custom element also show some fields 
(custom flexform fields), is this possible?

Kind regards,
Fabian