Sales Connect uses the Contact or Lead ID (also known as Salesforce ID), to help log information properly to Salesforce. There are a few ways that you can attach your Salesforce ID to your contacts in Sales Connect.
Push to Sales Connect: Using Push to Sales Connect will create a contact in Sales Connect with all the key contact information, including the lead/contact’s Salesforce ID.
Send Marketo Sales Email, Add to Campaign, and Click to Call Buttons in SFDC: When you use any of these three buttons in Salesforce, we will push the contact to Sales Connect with the Salesforce ID.
Import CSV: If you export a list of leads/contacts from Salesforce and you include a column with the Salesforce ID, you can map the field to Sales Connect on import.