1. The field_key can be ANYTHING you want but MUST start with field_
2. To generate a key in the same way that ACF does, use this:
$key = 'field_' . uniqid();
3. You can create function which returns an array (1 single field), and then call this function when you want the field. This is a way to include 1 field in multiple field groups. It will use the same field key, and that’s fine because the field_key is only used to load the field object and use it’s settings to modify the interface and value.
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