This all happens in GRUB, the bootloader. The 'init=/bin/bash' goes at the end of the kernel line (kernel=*)
When the Proxmox server boots and gets to grub, select the first profile in the list and press the 'e' key (edit), you will see three lines, the first tells grub where the boot partition (with kernel and grub conf) is located, the second tells it what kernel to load from that partition, and the 3rd tells it which initial ramdisk to use that contains a compressed set of drivers and system init items that is decompressed to RAM and used needed before the real root is mounted and programs can be executed from there.
Select the second line end press 'e' again, scroll to the end of the line and put your 'init=/bin/bash'. Press enter to save the change, then press 'b' to boot the modified profile.
Once you are dropped at a cmdline, run
Code:
#mount -o remount,rw
#passwd
and follow instructions to change the password. Easy, huh? This is why physical security of your server is important.
All of the changes that you make in GRUB at boot are temporary, the next time that you boot the 'init=/bin/bash' will not be at the end of the kernel line.