Danger Sidekick LX Reference Manual page 19

Press the trackball to select the New User button and advance
to the next screen. On subsequent screens, select the Next
button to advance.
2 Type your first name and last name.
Use the keyboard to type your First name, then roll the
trackball down to highlight the Last name field. Type your last
3 Select a username.
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name, scroll down to highlight the Next button, then press the
trackball to advance to the next screen.
The username you type will become the first part of your
email address (for example, username@example.com).
Before you select a username, read the following:
Usernames must start with an alphabetic character (A-Z)
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and can include numbers (0-9), dashes (-), periods (.), and
underscores (_), but no other punctuation. Although you
can use periods within a username, do not use a period
at the beginning or end of the name, and do not use two
periods (..) in a row.
Usernames must be at least  but no more than 3
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characters long.