[Typo3] Required selector boxes in extension

[Urs Weiss]

Hello

In my new extension i have some selector boxes which have references to
other tables and with an empty 1st entry.
Now i search a way how i can make the selector boxes "required". If the
empty entry is selected it should return an error message like it does
on textfields with "eval" => 'required'.
Already tried to set the value of the empty entry to '' but still the
same and as far i see there is no eval-config on selector boxes.

Greets
Urs