Perhaps this information will help you:
General function of Duress PIN is explained here:
https://grapheneos.org/features#duress
Regarding your question
Where to type the duress password
i find this post helpful:
Duress PIN/Password will work anywhere where the device credentials are requested. NOT JUST THE LOCKSCREEN.
Anywhere in the OS where the PIN/password is requested (enabling developer options, a PIN prompt to unlock an app, confirming your PIN to change the device's password, anywhere) can trigger the duress PIN.
The only exception is the SIM PIN prompt, since that's a different PIN, but we will likely extend it to that screen too, to try and avoid any confusion.
Usecase/idea for Duress PIN, found here:
Someone actually brought this up in our chat rooms, and I think it's a fantastic use of the feature. A phone snatcher is more likely to want to wipe the phone and sell it, which you as the owner kinda have to accept, but if they try to sign in to see if they can find anything that'll help them making more profit, there are two ways to combat the scenario:
The duress PIN could be a PIN that someone trying random common PINs would try. It could be a birth year that seems to be around your actual age (the thief might start guessing how old you were based on when they saw you and might trip the feature in this way), or something really common like "1234" or similar.
There are also other valuable contributions in the discussions i mentioned - it's worth scrolling through them.
Some reflections on GrapheneOS' Duress Feature can be found here:
https://dustri.org/b/reflections-on-grapheneos-duress-feature.html