That doesn't look hidden.
What I meant was how to get it on FMCs where those prompts like ALT APRT are not available. It was showed to me once and the trick is to press something on both CDUs at the same time. I don't remember the FMC version (U5.x U7.x ...) as the company had many leased a/c with different versions.
However, I very much appreciate your effort
Gaga1976