Elias here with Shopify Support. I'd be happy to lend a hand with this!
I understand you can't use Oberlo for your orders, as your products are custom designs, so you're fulfilling them manually. The Tracking URL can be the web address for the tracking page of your shipping carrier or the carrier used by your supplier.
When you're manually fulfilling an order, you can enter the tracking number first and see if our system detects your shipping carrier automatically. If our system detects the right carrier, you won't need to provide a Tracking URL. If the shipping carrier can't be detected, you can choose from a list of carriers for your tracking number, or if you don't see your carrier in the list - Choosing Other gives you the option to add the Tracking URL and link the tracking page from your carrier's website.
As another option, if you're not sure what your carrier's Tracking URL is, you can skip this and contact your customers directly as well. Notifying your customers about their shipment status is recommended, as it's a good business practice and can help alleviate some of your customers' concerns.
So that I can help find what the Tracking URL might be for you, can you confirm which carrier you're using? Or, if your products are fulfilled by your supplier, which carrier are they using?
Thank you for following up!
If the order is showing up in Oberlo, you can mark the orders as fulfilled or sync the orders from Aliexpress. If you're unable to sync the orders from Aliexpress, you can also add the tracking number to Oberlo manually (same guide from before). However, if the order is missing in Oberlo, then you'll need to try the solutions suggested in Oberlo's guide on missing orders or manually enter the tracking information on Shopify.
Though entering the tracking information on Shopify can help notify your customers of their shipment, it doesn't necessarily sync the order back into Oberlo. In order to sync orders back into Oberlo, that will depend on the issue that caused the order to go missing in the first place (eg. application bug, product is not synced with Oberlo). That being said, the missing orders guide helps cover the different scenarios for when an order doesn't appear in Oberlo.
I hope this helps! If you have any other questions on this topic, don't hesitate to let me know!