Put userid and password in an external source (e.g. an Excel file), and load their values in a Section Access table just like you load regular data from any other data source.
Note that userid/password can only be used for document security. Those definitions are local to that document only and will always present you with an authentication dialog upon opening the document. Switch to NTNAME-based Section Access to use LDAP/AD security where accounts are defined in a directory service and not in a document. Also allows you to use Single Sign-On.
