Where would I put this at in the filter?
The thing that takes salt is usually called a softener. The place where you put the salt is the brine tank, and the tank with the resin is called a media tank, among other terms.
============prototype procedure attempt================
A way to use Iron Out crystals to treat your resin periodically to deal with iron. This is not an authoritative procedure, but it should be pretty good.
A softener is not the best way to deal with significant iron, but it can be economical or cash-flow positive.
1.Dissolve 1/4 to 1/2 cup of Iron Out crystals into maybe 1 or 2 gallons of warm water.(X1: option -- maybe up the dose) Pour this into the brine tank, possibly down the brine tube. Agitate if you can.
2. Let this sit for 2 hours to 2 weeks. (X2 option: move the salt aside if the salt is shallow to expose liquid, and pour the IO into the liquid.
3. Start a regeneration. Usually after backwash, the brine draw cycle starts. Get to brine draw however it happens. Let the brine get drawn until the the brine tank is almost drawn. (X2) Alternatively monitor the drain line with a TDS meter, and look for a big increase in TDS. At that point, stop the flow of water with the bypass valve. That will let the solution sit in contact with the resin.
4. Let the solution sit in the resin for a time. That time (X3) might be an hour or might be 4 hours.
5a alternative: With an electronic timer, the cycling will continue while the water is off. However since the water is blocked, the solution says in contact with the resin. Then do a full regeneration to get the solution all cleaned out.
5b alternative: If you have an electromechanical valve, you can unplug the softener, and the softener will hold in the midst of the BD cycle. After you turn the water on, brine draw will continue, and you can slowly let the brine+IO interact with the resin for maybe 30 minutes. Turn the power back on, and the regen will continue and complete. That will get more efficiency out of the solution.
Note: various old posts have tried to describe a procedure, and I have tried to assimilate those. Go ahead and criticize this procedure. Different views can be helpful in tweaking a procedure. Or better yet, point to a writeup of the procedure that you like better.
There is not just one right way.
The X1, X2, etc are reference places to assist comment on times etc.