I add glycerin to mine. About 1- 1 1/4 oz per pint. Trapping supply stores have it or in a pinch I get the 95% pure stuff from the pharmacy. I don't add salt because of deer even though urine has some salt content on its own. If you use a trigger spray bottle the salt will rust the metal parts of the sprayer.
Coyote 1 nailed it; no salt, just a bit of glycerin.
If one is using pure urine, preservative is not needed, neither is salt. Pure urine won’t freeze!
This is incorrect. I use pure urine collected from the bladders of fox I catch. It will definitely freeze. Temps in the teens it will get slushy, temps in the single digits or lower it will freeze solid.