You just do it. No extra config required. Just do what the
servlet spec says - set the auth to FORM in web.xml and write the login form.
In WebLogic, Single Sign On between webapps is controled by the session cookie name (JSESSIONID). Those webapps that share the same cookie name participate in SSO with each other. I guess you could say they form some sort of "SSO group".
But it has nothing at all to do with how the auth is done (FORM vs BASIC).