how i pass an owners window messagebox.show opposite thread?

i am perplexing figure out messagebox( ownerwindow, ... ).

using reflection i ownerwindow defaults activewindow thread.

so wholly i need ownerwindow parameter arrangement another thread.

however i try this, i querulous threading exception.

    private vacant button1_click( vigilant sender, eventargs e ) {
        new thread( () => messagebox.show( this, "test" ) ).start();
    }

so looks wholly i need definitely state owners window, i cann't it!

code1