Your Shopify Domain is the text found after the URL of your store in the address bar. For example, in https://admin.shopify.com/store/6nzh0y-y6, the domain is 6nzh0y-y6.
You can also click on your profile in the top right corner, then select All Stores to view the list of stores associated with your account.
From the displayed list, your Shopify Domain is the value before myshopify.com.