Your subdomain is its unique identifier. Here are 3 ways you can find it.
Check your URL
If you have not enabled the host-mapping feature in Zendesk Support, your subdomain can be identified from its URL:
https://[ yoursubdomain ].zendesk.com
Check the Admin Home Page
If your Zendesk Support instance has host-mapping enabled, your Zendesk URL will be customized and may not include the subdomain. In that case, any administrator for your account can find your subdomain on the Admin home page . It is located in the upper right corner of the page, just below the user profile button.
In the screen capture above, the subdomain is support .
Request an Email
You can also get a list of Zendesk subdomains that your email address is associated with from this page, https://www.zendesk.com/login, where you will presented with this prompt:
Next, click on the text "Don't remember your company's URL?" Enter your email address at the prompt, and you will be sent a list of all subdomains associated with that addresss.