Access field name with space in config?

I assign field names in from a csv into columns, one of them is called "Primary Key"

I've tried a lot of combinations, but I can't figure out how to assign it to the document index.

document_id => "%{Primary Key}"

just %{Primary Key} appears, I've been searching for an hour now, thought I'd ask.

It was fucking case sensitivity, seriously, why does

%{Primary Key}

even work if it can't find the variable? For what reason should a programming language go "Oh, this variable doesn't exist, you probably meant it to be a string then".

This is not a programming language, simply a format string with some nifty variable expansion, the developers elected to have the expansion variable print verbosely if it did not find a match in the event keyspace, presumably so that having a %{ somewhere in the string didn't break everything