I had assumed you had that working already. You can't fill custom columns like this, you need to use "custom_columns_settings".
Either check previous page where similar questions were asked, or take a look at the well-documented plugin-defaults.ini, which you can easily open when editing your personal.ini. You should find this section:
Code:
## (Plugin Only) - You can also populate calibre custom columns with
## the site specific metadata using custom_columns_settings (but only
## if 'Allow custom_columns_settings from personal.ini' is checked in
## the plugin GUI config.) There are three parts, the entry name,
## then the label of the calibre custom column, then (optionally) a
## 'mode'. 'r' to Replace any existing values, 'a' to Add to existing
## value (use with tag-like columns), and 'n' for setting on New books
## only. (Default is 'r'.)
## Literal strings can be set into custom columns using double quotes.
## Each metadata=>column mapping must be on a separate line and each
## needs to have one space at the start of each line.
#custom_columns_settings:
# cliches=>#acolumn
# themes=>#bcolumn,a
# timeline=>#ccolumn,n
# "FanFiction"=>#collection