Step 1 - Copy Callback Url
1. Log into your Magento backend and go to Stores > Configuration > Fooman > Xero - OAuth2.0
2. Copy the url displayed in "Callback Url"
3. Click the button "Step 1 - Setup App in Xero" which will redirect you to the Xero Developer site (you need to log in if not still logged in to Xero)