How do referrals at Amazon work?

I am currently applying to a finance job at Amazon, in the bay area. I will use Rooftop Slushie for a referral (or I can DM you fine folks here too.) I filled out the job application (haven't submitted it yet) but there isn't a place to put referrals. How do referrals at Amazon work? Do I have to apply first and then the referral will use an internal website? Or does the person referring me have to send me a link first? Just want to make sure I understand the process before I officially submit my resume. Thanks!