The quoted template file looks like git nearly-master, and misses the fix that changes PALANG to $PALANG ;-) So please git pull the latest master and try again.
The script gets called with 4 parameters, as documented in config.inc.php: // Script to run after creation of mailboxes. // Parameters: (1) username (2) domain (3) maildir (4) quota // $CONF['mailbox_postcreation_script']='sudo -u courier /usr/local/bin/postfixadmin-mailbox-postcreation.sh'; This means the username is handed over as first parameter. The passsword will not be handed over (would be a terrible idea IMHO), so if you really need that, you'll have to query it from the database. However,...
It seems you are the first who wants a custom textarea field. Your struct looks good, but it seems you hit an implementation bug. Basically edit.php splits txtl fields into an array of lines (and does a few replacements) - and setmore() in AliasHandler.php combines the array back to a single comma-separated string. A clean solution will probably need some code changes, but I'm not yet sure what's the best way. (Not splitting the textarea to an array might sound like an obvious solution, but for example...
update for 3.3.1
You'll need two INSERTs for mailboxes - one for the mailbox table and one for the alias table. However, I'm not a fan of doing manual inserts ;-) I'd recommend to use the postfixadmin-cli shell command to create your mailboxes. See postfixadmin-cli mailbox add help for details. There's even a trick to carry over your pre-hashed passwords: temporarily set $CONF['encrypt'] = 'cleartext'; when handing over the password hashes to postfixadmin-cli so that they don't get hashed again, and don't forget...
The "403 forbidden" comes from your webserver (apache or nginx), you'll probably need to adjust its config. Have a look at the apache/nginx error_log, it should give you the needed hints to get this fixed.
What GingerDog said ;-) Additionally: Typically the database only has the domain.tld/username/ part of the maildir path. The /var/vmail/ part is probably in your postfix and dovecot config, you'll need to change it there.
That symlink is still the wrong "fix", and makes your setup less secure. templates and templates_c are two completely different directories: templates contains the template files shipped with PostfixAdmin (read-only) templates_c is for the template cache files generated by smarty (written on-the-fly by the webserver/PHP while using PostfixAdmin). Please get rid of the symlink again, and simply mkdir templates_c. Note that templates_c needs to be writeable by the webserver, so you might need to chown...