NOTE: A simple way to look up the Shopify domain is to navigate to your Shopify store admin. Once you log in you will see the domain in the web browser URL address bar. It is the name following "store/"; for example, in the URL https://admin.shopify.com/store/toms-fashion, "toms-fashion" is the Shopify domain (i.e., toms-fashion.myshopify.com).

Alternatively, you can look up the domain in the store domains section. In the main left menu, navigate to Settings > Domains, then you will see an overview of your store domains. The primary domain is the one that all of your other domains are automatically redirecting. This domain can be easily changed. What you are looking for is the Shopify-managed domain that ends with "myshopify.com". See the screenshot below for reference.