For the lazy, this is how you can do it sans extension, but for the community's sake, an extension would be much better.
1) Create a freeform mode behavior. See attached screenshot.
2) Paste the source in.
https://raw.github.com/bsinky/gjhaxeAPI/master/api/GJAPI.hx3) Use it using a tiny bit of code. For example, to submit a score, do something like this (see their code to see what the exact parameters are to pass in).
var api = new api.GJAPI();
api.addScore("MYGAMEID", "100", "DESC", "ASDFGHJ123");
I don't know exactly how this API works or what it wants, but they (as in GJ) should have some docs on this.
The nice thing about this is that this should work on all platforms. It isn't restricted to Flash at all.