Seated Good-mornings : extended or flexed spine?

Sheiko says :

2. About seated good mornings: If you never do these, try to begin with 40 kg. If that is easy add 5 kg every week. Try to do long amplitude movements, your chin has to touch bench.

There's no way I can touch the bench without flexing my back. With a fully extended back, I can barely go as far as 45degrees to the bench.

Does this mean seated good-morning are supposed to be done with a flexed back ? What about standing good-mornings?

Examples of well done seated goodmornings:

https://www.youtube.com/watch?v=NwC3fzWhnPg

And same good doing really beautiful standing goodmornings:

https://www.youtube.com/watch?v=TWV5Yc6HqUc

There's no way I can touch the bench without flexing my back. With a fully extended back, I can barely go as far as 45degrees to the bench.

You will improve with time. Apparently you are not flexible enough at the moment.
Most important is that back should be straight.

You can do at the end of your training some https://www.yogajournal.com/poses/wide-angle-seated-forward-bend just 4 reps 1 min.
In this pose you can also ask someone to stand behind your back and very gently push your back.

Thanks guys!