Echoing what Sanford Whiteman said, if you're integrated with SFDC, you won't be able to successfully make salesperson API calls. My opinion on this, is to let Salesforce manage the lead distribution rather than having a smart campaign in marketo do it. Have a custom field or even a utm save the email address of the sales owner and then set up your lead assignment rules in salesforce. two main reasons: 1. if you have many reps, the choices in a marketo smart campaign can get messy. 2. sales people stuff is easier managed in one place, e.g. when someone leaves or territory changes and you have to mass migrate accounts, update lead assignment rules etc...
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