This is a good question. You don’t need to register or transfer a domain name to Alibaba Cloud to enjoy/test their free trial offer unless you are specifically deploying a website that demands for the same.
If you are starting out, you can try their service with the public IP address assigned to you(e.g. when you create an ECS instance)
Also, there are over 40 products that you can try on Alibaba Cloud without necessarily using a domain name for instance. you can provision an ApsaraDB and connect to it without requiring a domain name. They usually assign a free hostname for you to use.
Other products that you can try without a domain name include installing software applications like Apache, PHP, MySQL, WordPress e.t.c
So, you simply use the public IP address and only add a domain name when moving to production.
You can see how the Alibaba Cloud free credit is split among their products.