Hi Isabela,
Does your SFDC instance have a field called Comments or Notes? If so, simply having that field in the form will sync that data into SFDC when the form is submitted. Otherwise, I would create that field against your lead and contact records in SFDC and then pull it into your form.
Or, is what you want is to add the Comments field into your SFDC task that you are creating from Marketo, then you would add the token for the field within your task detail, eg {{lead.field name}}. This would add any values against that field to the task in SFDC.
Hope that helps. If I have not understood correctly, please let me know.
Thanks
Natali