The name that you reserve with your pre-order is one of the three accounts that you can create with your Key Code (that is, you can create two additional accounts when you get the final game box and your Key.)
Here are some general guidelines for registering your account name:
You can use a-z, A-Z, 0-9, - (dash), _ (underscore), and . (period) in your account name.
You don't need to include your clan tag, as noted below, this can be added via the menu later.
You don't need to include color codes in your account name - you can color your name from the options menu using the 32 available color codes.