In the next step Step 5: Create DocuFlow Connected App in Salesforce you need the DocuFlow OAuth Callback Url. This page describes how to open the DocuFlow Settings page where you can get it.
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Step by step
Search for DocuFlow Settings in the App Launcher:
Press Settings:
The DocuFlow (DocuBuilder) login page will start in a new browser tab. Fill in your Username (Gebruikersnaam) and Password (Wachtwoord) and press Inloggen:
Choose Salesforce > Settings in the menu on the left side and keep the page open while you continue with the next step: Step 5: Create DocuFlow Connected App in Salesforce