Two fields are enough — but the domain must be complete
The Shopify connection asks for two things only: the store domain and an Admin API access token. The domain must be written in full — not a shortened form, but examplestore.myshopify.com. Even if you use your own domain (examplestore.com), the technical myshopify.com address is what goes here; the two are different, and this is where people get stuck most often.
The access token is generated under Shopify Admin → Apps → Develop apps, once you create your own custom app and grant it Admin API access. The token is shown only once; if you miss it you have to generate a new one.