As a seller, you can't "opt-in". You get asked.

Amazon has some random criteria which they feel is a good idea not to share with the sellers. They first called us several times requesting us to start FBA but we just couldn't due to the nature of our products, it isn't feasible for us to do FBA. Anyway, after months of rejecting their request to enroll for FBA, they offered us SFP (Seller Fulfilled Prime).

Spend a couple of months selling on Amazon and ensure you have an excellent track record of shipping orders at a great speed and you should see them offering you the SFP as well.

Good luck!