When you choose to create a Zap using Better Proposals, you'll be prompted to connect a new account.
Click the button and you'll see a pop up that asks for your API key. You can find this in Settings > Integrations > Zapier
Paste your API key in and click the button to continue. The pop up will close and you'll see that your Better Proposals account has been added and is ready to connect.
Click the 'Test' button and once it has connected successfully, the button will turn green and you can continue to create your Zap.