Important Note: Google Shopping Actions supports inventory values via the attribute "sell_on_google_quantity". By utilizing the below Modify Products attribute, your feed will automatically be eligible to send this attribute.
However, you must have inventory values for your products in your store - such as when setting products to "track inventory" in BigCommerce, or when setting "Track quantity" in Shopify.
The same feed you send for Google Shopping can also be used for Shopping Actions. The Feed Tool add-on for Shopping Actions will allow you to adjust which products you wish to include specifically in Shopping actions.
Once purchased, you can use Modify Products and the Included Destination attribute to do so:
As a Filter Only rule, the value that Google requires will be sent automatically without you having to set the destination.
You would simply use a Filter to focus on specific products you wish to send also for Shopping Actions.
Use any of the filter options available within the list and set a value, such as specific IDs entered one by one.
Once satisfied, you'll have to Save the rule and then under Feeds, send your feed to trigger Google to crawl your changes.