Moira has different clients. Other than making queries directly, the main clients are the command line clients. In the case of hostnames or subdomains, the best clients are stella and simply moira the command.
Ways of acquiring subdomains
At least when it comes to SIPB, subdomains are tied to machines, which have a specific hostname which is a subdomain of MIT.EDU. If you want to use an external IP address, or an external service such as GitHub pages, contact the service desk.
Machines are tied to networks, or subnets. Some examples include SIPB-460 (the SIPB machine room), W20-PRIVATE (stud, but private IP addresses), W79-PUBLIC (Simmons), 62-PUBLIC (East Campus), and so on. XVM has its own network SIPB-XVM.
Machines also have an IP address, located inside of the corresponding network. If you claim the corresponding IP address using static IP address configuration, and if your machine is located in the correct network, then you can act as the corresponding machine and will have the hostname (for Lemonpepper, the network is SIPB-460 since it is in the SMR).
Each machine can have up to two aliases or cnames (there are exceptions to the rule, for example scripts and IS&T's sites.mit.edu pages can have many aliases).
Creating a new machine
The Hardware Operations (HWOPS) project is the authority to make machines and IP addresses in SIPB-460. Only they can do so. In the case of machines in other networks, such as dorm networks, ask IS&T via this form: https://ist.mit.edu/network/ip-request. For other requests, ask the IS&T service desk.
Make sure to ask the person who made the machine to set a moira for the project as owner. The owner list will act as access control, and will be allowed to make modifications, such as adding aliases.