They will try and leave it....I want them to be the benefits of FF.
How long has this been going on?
Many animals will try just a little bit of new feed, then wait a while to see if it makes them sick, then eat a bit more the next day when they have decided it really is safe.
So you might just need to offer small amounts for several days, so they can learn it is good to eat.
You mention the benefits of fermented feed. You can get some of those by just serving the feed wet. (That keeps the chickens from scattering dry feed all over, so it reduces waste and saves you money. And many chickens like the wet food better than dry food.)
If you really want them eating fermented feed, maybe start by serving wet feed, and if they eat that you can serve wet feed that is a little bit fermented. If they eat that too, then serve them food that was fermented a little bit longer. Work up gradually, until you are serving it at the level of fermentation you want, and the chickens are used to it.