You don't need to apply for it, it is added automatically
How do I apply for HSC Plus?
No separate application is required. For admission to undergraduate programs at UNSW all applicants completing an Australian Senior Secondary Certificate [Year 12] - either in Australia or off-shore - must lodge an application via the Universities Admissions Centre (UAC). Applicants completing the International Baccalaureate in Australia also apply via UAC.
If you have a preference for a program included in HSC Plus at the close of change of preferences AND your performance in a Year 12 course or courses is in a specified band, bonus points will be automatically added to your UAI or equivalent to increase your selection rank for that preference.
Source:
UNSW
No that isn't how it works. If you are eligible for it they will 'add it' to your uai to be used when selecting entry. i.e. they artificially inflate your UAI for selection purposes.
There's examples of how it works on their website if you go to the link above.
Educational Access Schemes (EAS)
It is for applicants who have experienced long-term educational disadvantage.
See here:
Universities Admissions Centre (NSW & ACT) Educational Access Schemes