Hi @Lotus Pacific Group Ltd,
As of the moment, there isn't an option to add a custom field or create a sub-project of your created projects.
@Rustler is correct. In addition, you can manually enter all the additional details of your projects in the Notes field. I've added a screenshot below for your visual reference and an article you can read to learn more about projects: Set Up and Use the Projects Feature.

Meanwhile, may I ask for more details about the official QuickBooks agency in Vietnam you say? Any details you can share will be appreciated so that I can also share all the accurate information. In any case, I've added a few links below for your reference:
If there's anything else that I can help you with or if you're referring to something else, please let me know in the comment section down below. I'll be always around ready to help.