Get value out of key from document

Hi

I don't understand the filter possibilities yet:

I have output documents

{
"xml" => "some xml here",
"@timestamp" => some date here,
"@version" => 1
}

This is my filter - the input is not important. What I want is to put the xml field from my document through the xml-filter to parse the xml to json (which happens automatically with the xml filter).

filter{
xml {
source => "message"
target => "chambers"
}
}
output { stdout{ } }

So how do I address this xml field?

The xml filter's source option should contain the name of the field containing the XML you want to parse.