In the example that didn't work, do you want to extract Referer too? Or do you only care about host, connection and accept? Are the values always in the same order after the headerValue= part of the event? Not sure if just hardcoding fieldname is an option or makes sense.
In general though, there are probably a few ways to make this work for you. It's just a regex thing. For example, something like this might work in props.conf