Contacting potential postdoc advisor in open announcements
Apply via the university system first - most likely you will have to do that for HR to even consider you a candidate. Then, reach out to this person and ask him or her if they can and want to receive a copy of your proposal. Some universities have rules when there is an open announcement regarding what they can receive from candidates and how they receive it (standard email versus the online submission system). Plus, if the person is swamped with work, getting an unsolicited research proposal via email may be overwhelming for him or her and not helpful for you. If he or she wants to work with you and knows that you applied, they can advocate to administration and/or committees about your merit as a candidate.  Good luck!