When fulfilling an order at Shopify, you are not able to choose a carrier other than the default carriers (like UPS, DHL, USPS). Sadly, there is no settings page to allow you to change the carrier list. You will have to choose "Other" and then input the tracking number in the following format: - slug#tracking number, for example, skynet#238722440806
The slug for Skynet Malaysia is skynet. You can find the slugs from this page - https://www.aftership.com/courier/
One thing Shopify does is that it allows you to modify the tracking URL in the Shopify fulfilment email notification if you choose "Other" as a carrier.
AfterShip supports tracking of over 700 carriers.