http://j5kv55yiu3rneydqn4f35kyhejbq3fhkkm7glg5nbtvxgaxthou7g2ad.onion/start-here.html
If you don't have a Proton account, head to https://proton.me to create one. In Hush Line Settings, select the Email & Encryption tab. You'll see a search input with placeholder text reading "Search Proton email..." - enter the address you just created and click the Search Proton button.