Maybe I'm weird but I always launch TERA in steam from the system tray or a shortcut so I don't have too open up the steam client.
The problem is that you will automatically be connected to the EU servers and won't get the window to select your region.
So I found a useful workaround.
Add this to the launch options in steam and you'll automatically connect the the NA server instead:
--authType steam --baseUrl https://na-login.tera.gameforge.com/
Right-Click TERA in Steam library->Properties->General->Launch Options
Unfortunately you won't be able to launch to the EU servers at all until you remove this but that's fine for people who just play on NA.