You send it a WoL packet[0], use key-based SSH to log in to the initramfs environment[1], and type in your password. Or if you have a TPM you can just stick encryption keys there. Do note that if the device lacks secure boot or such, this is vulnerable to an attack where the initramfs is modified to steal your password; how bad this is depends on your threat model.

[0] https://en.wikipedia.org/wiki/Wake-on-LAN

[1] https://askubuntu.com/questions/1269981/unattended-headless-...