Few things;
1. When you apply for beta invite, you have a choice to post your cdkey, if you have beta you have no legit cd key. There is a small chance that you can get in beta without legit cd key but it suppose to be higher odds if you have legit, full cd key.
2. This is only a Beta though, so even if you get into the beta, when the game comes out full, you most likely will still need to buy it. Thus getting a legitimate cd key that way.
3. Warband is, as of right now, completely separate from Mount and Blade. Most likely since it is a Beta, it gets its own folder and its own launcher. So you do not need to do anything special, such as hitting a certain level, in regular Mount and Blade to play Warband, just need to get invite.
Edit: Arrr someone beat me to the answer.
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