Xilium.CefGlue how to execute JavaScript with return Value?

Xilium.CefGlue how to execute JavaScript with return Value?

I'm quite new in programming multi-threading and I could not understand
from the xelium example how I could execute a javascript and get the
return value. I have tested:
browser.GetMainFrame().ExecuteJavaScript("SetContent('my Text.')", null, 0);
the javascript is executed, but I this function don't allow me to get the
return value. I should execute the following function to get all the text
the user have written in the box..
browser.GetMainFrame().ExecuteJavaScript("getContent('')", null, 0);
the function TryEval should do this…
browser.GetMainFrame().V8Context.TryEval("GetDirtyFlag", out
returninformation , out exx);
But this function can't be called from the browser, I think it must be
called from the renderer? How can I do so?
I couldn't understand the explanations about CefRenderProcessHandler and
OnProcessMessageReceived.. How to register a Scriptable Object and set my
javascript & parameters?
Thx for any suggestions how I could solve this!