Generate Random Password Including login username

your code is already correct, if you just want to append some random things with the username, keeping the username untouched then just modify the code to not touch the username and just generate the random string and later append it with a username.

protected string RandomPassword(int length)
{
string validChars = "0123456789!@#$%^&*?";
Random random = new Random();
char[] chars = new char[length];
for (int i = 0; i < length; i++)
{
chars[i] = validChars[random.Next(0, validChars.Length)];
}
return new string(chars);
}
protected void btnSubmit_Click(object sender, EventArgs e)
{
int length = 10;
log.Debug("Entering btnSubmit_Click");
string NewPwd = lblUserNameData.Text +RandomPassword(length);
}