It can't be done. It is a functionality that disappeared when writing the callback system of tgui v0.6.
I am planning to allow it again, but this will require me to manually check every widget agains every different kind of callback that it has, so its a pretty big change (lot of copy pasting though).
I am actually waiting until the new form builder is ready. It will allow me to make a few more simplifications to the loading from file, giving me slightly less work with implementing this. The only problem is that this can still take some time. I really hope to get it finished before september 20th though, afterwards I won't have so much spare time anymore.