Protocol based authentication:
-ab "userid:pwd" basic authentication mode
-an "userid:pwd" NTLM authentication mode
-ad "userid:pwd" digest authentication mode
-ak "userid:pwd" kerberos authentication mode
-aa "userid:pwd" automatic authentication mode
Web based authentication:
-am {macro path} web macro authentication mode
Chris, if you want to authenticate via HTML FORMs then use a web macro. When you record the macro you will have defined the FORM fields that it is looking for. Another method is to setup your authentication fields with their corresponding values in the Web Form Editor (Tools->WebFormEditor), but that's an ugly way to do things (macro = better).
Have a look at this page, it explains basic authentication well: http://httpd.apache.org/docs/1.3/howto/auth.html
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