When setting up your GoCardless integration, you will be prompted to enter the direct debit mandate URL into GoProposal.
Getting The Mandate Link
Retrieve the mandate URL by going to Customers > Add/New Customer. You should find a link in the format 'https://pay.gocardless.com/...'
From GoCardless for Xero
Copy the mandate URL from your GoCardless for Xero account by going to Settings > Xero Settings > Xero Automation.