Adds a Product

If you have trouble with creating a Product, try creating Products via the web interface, and then access those Products via the API. This way you can see examples of the fields and sub-elements attached to the Product. A common source of problems during write/update request is caused when users omit required fields, or insert invalid data in a field (e.g. reference a CATEGORY_ID that does not exist in the user's Insightly instance.