Hi,
yes, it's not possible to start in fullscreen, it's has flash security reasons,
switching to fullscreen can be only done in response to a user event (like a click or a key)
here a quote from the Flash/Actionscript3 Documentation -
Full-screen mode security
The ActionScript that initiates full-screen mode can be called only in response to a mouse event or keyboard event.
Users cannot enter text in text input fields while in full-screen mode. All keyboard input and keyboard-related ActionScript is disabled while in full-screen mode, with the exception of the keyboard shortcuts (such as pressing the Esc key) that return the application to normal mode.
A message appears when the content enters full-screen mode, instructing the user how to exit and return to normal mode. The message appears for a few seconds and then fades out.