The username and password fields are for entering in your username and password that was assigned to you or that you set up when you got your optonline email account. It is the same username and password you would use to log into webmail.
Yes if that is what you use to login to webmail. Best thing to do is check your email settings on your computer to be sure. Some services use the whole email address some use the portion before the @ sign.
If your talking about a username you would just use example for opt mail. [Settings for opt mail|http://optimum.custhelp.com/cgi-bin/optimum.cfg/php/enduser/popupadp.php?p_sid=undefined&p_lva=undefined&p_li=undefined&p_faqid=247&p_created=107 6619294&psp=undefined]