I have set up a panel that shows me, between other things, a "book page". I need to add a link like the "add child" one that is given by default, but with a few other options.
In panel I'm able to add a "custom content" with an html link, but I can't add the "&parent=" information with a substitution.

I would say that it should be %node:book:mlid (because that is what the book module is using), but there is no such things between the options.