Here Products previously loaded into the store can be attached to the app as long as the SKUs match the T14 part numbers. The simplest way is to edit the product and set the "Inventory managed by" option to "Turn 14 Distribution API" like this:
But that's not great if you have hundreds of products to set. In that case the Shopify bulk editor is a good option. Go to the Products>Inventory section of the Shopify admin:
This is under the Inventory section of the Shopify admin:
Then sort by the inventory location you want to change (this might be Shopify, or the address of your store)
Select all products then "Select all 50+ inventory in your store" and click Edit Variants:
Add the "Fulfillment Service" option to the columns, then multi-select all of the products you want to move over to the app (shift + select all). After this is done you must save the edits.
Here is a video that demos this process.