Marketo needs the Organization Service URL to sync with Dynamics On-Premises instances. Here is how to find it in Dynamics.
1. Log in to Dynamics. Under Microsoft Dynamics CRM, select Settings.
2. Under Settings, select Customizations.
3. Click Developer Resources.
4. The Organization Service URL can be found under Service Endpoints.
Copy and paste this URL to Marketo, and enjoy the rest of the sync.