French winsurfer here (4 years in NSW, relocated in 2014) and 3 time Defi entrant (2009,2010, 2015... 2016). The 'competition' part is really important*. So, what seanhogan said, basically.
If you want in, sign in on the first day (next monday 10 am paris time). While it might not sell out in 24h, you're not the only one to have seen those videos... I reckon it might sell out in less than 5 days.
For more details on doing the defi from an aussie point of view, get in touch with izymiester who did it last year. I also have a uni mate coming this year, so you won't be the only australian joining in on the fun. Keep in mind that chance of wind during the defi are 50% (but if there's wind, it will be over 25kt). You can mitigate that by staying longer. I'd say 75% change of wind over a week, and 95% over two.
*Context: In France, when joining a sport club, you need your GP to sign you off for that sport at the beginning of the school/uni year (september). And if you want to enter competitions, the GP shall have mentionned that you were fit for competiting in that sport. It is up to the GP to decide which tests are required for him to sign you off for a particular sport. For sailing most GP would check your joins, and quickly look for any heart/respiratory and ENT (Otolaryngology) region issues.
The defi is a bit picky, so they want your GP to stamp that piece of paper, as well as sign it. You'll also need it to be dated (and the date shall be after the 8/05/2015).
EDIT: When signing up you will have to select the temporary sailing permit which will add 26 euros to the price.
Full details from the organisation:
www.windmag.com/media/D%c3%a9fi%20Wind/2016/Docs%20pre%20event/Defi%20Wind_Terms%20&%20Conditions.pdf