It's a property, not a serialized field. Right now it can only be set via code. The reason for this is that the property is a single object but the MoveTowardsLocation returns an array so it's easiest to set via code to reduce confusion.
So, I seem to be back to square one here. I've tried the code snipped you posted but still no luck. I have a very small simple scene setup, two cover points, but the AI just ignores the cover positions no matter what I seem to try.
Will the AI use cover positions during combat phase?
The PredeterminedMoveTowardsLocation should point to the Transform that the MoveTowardsLocation component has been added to. The StartCover task uses this to tell the AI agent what cover position to take.