[Luks/DM-Crypt] Ask for encryption password on SSH login?

[Evils]

Hi everyone,
first, sorry for the bad topic subject, I hope it's halfway decent.
I got the latest Arch Linux running on my Raspberry PI 2 which currently has two external drives attached.
One of these drives is encrypted with dm-crypt in the LUKS container format.
My raspberry pi is mainly used with NZBGet, a Usenet downloader. Nzbget starts automatically with systemd on system boot and requires it's temp directories to be existent on startup.
Now as my encrypted drive is holding these temp directories and it happens from time to time that I need to restart my raspberry pi, I was wondering if there's a configuration setting/dependency option on my nzbget's systemd file so it'll hold off till my external drive is available?!
In addition to that, I was wondering if there's a setting so I get automatically asked for the mount password of my external drive once I login through SSH after a fresh reboot if the drive isn't mounted?!
Thanks for your help,
Evils