Warning: I am not an Unraid Expert an my Knowledge of auto array startup decryption and Unraid events depends on the mentioned article below and some StackOverflow entries. This configuration and settings might not be the most secure settings you can apply for this scenario. It works for me for now and I am still learning and improving my setup. The configuration is oriented to the suggestes way of an Unraid Community developer combined with try-and-error of myself.
tested Unraid Versions 6.8.x, 6.10.x and 6.11.x - it should work with Unraid Versions >=6.4 Use Case: decrypt HDD in array by exchanging keyfile from another Unraid server in the same network. File to change: /boot/config/go (on the system where you want to decrypt HDD)