There is no default password, not since the v4.6.1 was released. See
https://www.qbittorrent.org/news. Now, since we don't actually get to see the console during installation to TrueNAS, we're in a bit of a pickle.
One suggested solution is to edit the /var/db/qBittorrent/conf/qBittorrent/config/qBittorrent.conf and set the value
WebUI\Password_PBKDF2="@ByteArray(ARQ77eY1NUZaQsuDHbIMCA==:0WMRkYTUWVT9wVvdDtHAjU9b3b7uB8NR1Gur2hmQCvCDpm39Q+PsJRJPaCU51dEiz+dTzh8qbPsL8WkFljQYFQ==)"
Which is supposed to set the password to 'adminadmin'. Didn't try it myself though.
Or, if you had a backup of a previous setup or if you are doing a migration, like I did, you can copy the WebUI\Password_PBKDF2 and WebUI\Username values.
How do you edit the file you ask. Well, from within the jail, not that easily. I copied the original out of the jail, edited, copied back into jail. The first step would be to shut down the jail, then copy:
cp /mnt/<poolname>/iocage/jails/<jailname>/root/var/db/qbittorrent/conf/qBittorrent/config/qBittorrent.conf /mnt/<poolname>/<sharename>/<locationofchoice>
Replace the <> values according to your own situation.