Exactly here! The documentation is divided into three parts, which you can download by clicking on the corresponding button.
The Core API documentation explains user management and accounting, and the Domain API documentation explains domain handling.
If you also want to offer your end customers their own domain management, you will find information on this in the DomainEdit API.
You can find the access data for this, including base URL, on your access data sheet of your domain reselling product in the myhosttech customer center under “Servercenter > Servercenter” in the Solutions tab.