Custom domain name option is available both for Bitrix24 on premise and cloud versions.
For Cloud accounts, there are several options for administrators (read more here):
When an account is created, it is given a random name such as b24-aokwy2.bitrix24.com. You can customize it.
Switch to your own domain
Use your own domain name for your Bitrix24. For example, use my.intranet.com or mycompany.com instead of intranet.bitrix24.com.
Rename Bitrix24 3rd level domain 3 level (*.bitrix24.*). For example, from example.bitrix24.com to test.bitrix24.com.