It's not supported. Instance variable names are deliberately stripped out to impede reverse-engineering (a common concern amongst users). They are sometimes available in preview mode for the debugger, but it will stop working when the project is exported.
This seems to be a half-way point to allowing arbitrary expressions anyway, so it seems you might as well just make it fire a trigger which can set a return value, and then you can use any expression at all in a placeholder, including instance variables.
I'd also point out that you'd better implement that carefully or it will be a playground for injection attacks!