Snake Oil...marketed to empty chicken keepers pockets of money.
I prefer to season my birds just before I cook them and not a minute sooner.

Good feed, water, and space is all they should need to have healthy immune systems.
Are you having specific problems that you think this will help?
If so, please explain.
Oh, and
Welcome to BYC! @Fusezel
Where in this world are you located?
Climate, and time of year, is almost always a factor.
Please add your general geographical location to your profile.
It's easy to do, (laptop version shown), then it's always there!