I think you would use the “Send Admin API request” action and call “inventoryAdjustQuantities”. Details of the inputs are here: https://shopify.dev/docs/api/admin-graphql/2024-01/mutations/inventoryAdjustQuantities
One note - be careful that you don’t create an infinite loop by running on inventory updated and then update inventory. You should check both the before and after quantity to prevent it from calling the action repeatedly.