For the closed test, you will be using your Steam account to play the game. There is no need to sign in, just get in the game, choose a server, and start your shift! Your name will be automatically set to match your Steam name.
Please note that information is not shared between the EU and NA servers, so unfortunately you will have to replicate your perfect K/D/A ratio on both by playing.
After the test is over, all progress will wiped before the next test, so try to avoid getting too attached to your account.