So, there are tons of Google results on this and I finally found one that actually works.
Trying to get the field keys programatically is a waste.
Simply go into the Custom Fields back end, scroll down the field options, check the Show Field Keys box and copy and paste the keys into the code.
I’m sure there’s some reason this is kind of confusing, but the fact that this simple thing isn’t in the documentation is a bit incomprehensible. Your documentation says use the field keys, great, it doesn’t say HOW TO GET THEM….
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