It is reccomended to fill and encrypted data container with random data.
You can however if you are in a hurry use a sparse file.
dd of=crypt bs=1 count=0 seek=900G
It will not allocate the space immediately. We can fill it later.
losetup /dev/loop0 /path/to/crypt
cryptsetup luksFormat /dev/loop0
cryptsetup luksOpen /dev/loop0 crypt
If we fill the mapper with zeros it will write random data to crypt (Really?)
dd if=/dev/zero of=/dev/mapper/crypt