Or anywhere else that we can bring up.
It's easy to miss the text message that mentions it at the start.

Use tha latest test patch(P10)
Sorry that i'm to lazy to search for it.

Yeah, that's already implemented in one the test-patches. Just get the latest one (currently P10, should be P12 in a pretty short time).

So it's only mentionned in F12 when the pit stop is mandatory?

It is mentioned at race start, and additionally you can look it up in the F12 menu (<- new). Btw, P12 has been released.