How to send a value for [Single on/off checkbox] using form api
SOLVE
Thanks for weighing in, @benvanlooy. I agree this is a bit strange.
For others viewing this topic, if you're ever unsure of the format of a property's options, the "Get all [object] properties" endpoints are the best place to check.
For example, the Get all contacts properties endpoint returns two options for each property with the "fieldType": "booleancheckbox":