Icingabeat custom vars

i have an icingabeat test working but do not see any custom vars from either host or services - is there a way to add them to the icingabeat? in some other api methods they’re listed as a “var” dictionary but i can’t see any reference to it in the icingabeat docs.

documentation says it’s possible to add a filter which includes a join but i have not been able to get that to work either.