Check "Recorded Login Sequence" if you have one for the website, this is, for the majority of the cases, the most reliable option for authenticated scanning.

A login sequence may be recorded using AppScan Standard's GUI by following these steps:

"Scan" > "Scan Configuration" > "Login Management" > "Record" > [ record your login...] > "I am logged in to the site" > "Details" (Tab) > "Export" (small icon on the right side).

Check "Form Based Authentication" if you do not have a recorded login sequence, this option will require an user name and password combination and is not guaranteed to work for all scenarios.