on the login page, edit the Security Authenticate user server behavior. make sure the redirect on success setting is left blank, then go to the session tab and note the name of the ID session that gets created.
open the bindings panel, expand the session collection and make sure there is a binding for the ID session, if not create one by clicking the plus button, select session, and name the binding the same as the session you noted in the session tab of the authenticate user behavior.
Now add an insert record behavior, on the server behaviors tab, click the plus button and select webassist -> data assist -> insert record -> insert Single record.
select the log table and bind the log user ID column to the ID session variable created by the login.
make sure that the insert behavior code is after the authenticate user code. it will probably be added before, in which case you will need to manually re arrange the code blocks.