You should first create new fields in Admin - Field Management, you can call them similar to the utm parameters. Then you add these fields to your forms and set them up as hidden fields (under Input type) and specify these collect values from URL (Get Dynamic Value) and enter parameter name.
Hidden fields will now pass defined parameters values from URL to hidden fields in Marketo.
I'm Daynamics user but logic should be same to Salesforce - you create this same fields in SF and include them in Sync between Marekto and Salesforce.
Sara, Thanks for the answer. I'm not sure I fully understand what to do. I created a new field - utmBlog for links that come from my blog. I entered the field in the form and made it hidden. Not sure I understood exactly what to do from that point...
I included hidden fields to capture utm parameters for forms within Marketo landing pages that we're driving traffic to through paid search. But, I'm getting quite a few blank sources coming through. Thoughts?
I'm also receiving quite a few default ("no source", as an example). Thoughts on what % of overall form fills I should expect to see blank and default values?