If you want dry seasonings to stick to popcorn, you will probably need to add a liquid to adhere them with.
You could try adding butter or oil to your popcorn while it is hot, then adding the salt and tossing it together.
If you're avoiding extra fat, a few spritzes of a non-stick spray (like Pam) might do the trick without adding significant fat.
cooking.stackexchange.com/users/16863/SourDoh
I air pop my popcorn and then I spray it with Dr. Bragg Liquid Aminos .
It has a salty flavor and if you spray it lightly it doesn't make the popcorn soggy.
If all you want is the salty flavor then this will do it
but it also has the added advantage of allowing other spices to stick as well.
cooking.stackexchange.com/users/6326/DQdlM
In addition, as Sourd'oh says, to using some fat as glue.
Water will also work if you don't want calories, but will soften the popcorn somewhat.
Use popcorn salt.
This is salt that is very, very fine, so that it sticks more easily to popcorn.
Additionally, applying the salt immediately after popping when the popcorn is very hot will help.
cooking.stackexchange.com/users/14401/SAJ14SAJ
Put the salt in a spice grinder (such as this one )!
My grinder has several options for granule size.
Unfortunately, it has no markings or logos on it
so I can't tell you what the exact type it is else
I'd highly recommend it.
cooking.stackexchange.com/users/19492/Zmart
The problem is not liquid.
The problem is the size of the salt.
You have to grind the salt.
If you don't have a spice grinder, (I use a mortal & pestle)
you can put a cup of salt in a blender :) and use it in the future.
cooking.stackexchange.com/users/70190/mandddd
Why not add the salt to the oil while it is heating up - or otherwise mix with the kernels before throwing them into the oil?
I have done this both with sugar and salt and it works well for me (and the salt does not appear to burn or damage the saucepan...
If you shake everything up (as you should do) while cooking, then the popcorn gets evenly coated.
However, no-one else seems to suggest this, so there may be something I am not considering...
cooking.stackexchange.com/users/75597/Peter Bright
I've been popping my own corn and I want to season it with salt, but when I sprinkle it over the popcorn it just ends up at the bottom of the bowl without taking to the popcorn. How could I get the seasoning to stick to the popcorn?
How do I get seasoning to stick to home-popped popcorn?
cooking.stackexchange.com
<seasoning><popcorn>
cooking.stackexchange.com/questions/35762
cooking.stackexchange.com/users/9605/Danger Fourpence
You probably need some kind of liquid for adhesion, which is usually butter or oil but can be liquid aminos or water (which will also soften the popcorn). Try using popcorn salt or adding the salt immediately after popping.  In order for salt to stick to popcorn, the popcorn must be wet or hot. Methods include andding butter or water or adding the salt when the popcorn is still hot.
Some kind of fat or liquid is needed to make the salt stick to the popcorn, for example butter or water. Using popcorn salt or adding the salt when the popcorn is hot helps it to stick.