If you have a program you are using to track visits from your online ads and have configured clicking the free trial button in the landing page as a Success progression status, then you should be able to see how many of your new leads from your online ads converted to a customer, using the First Touch (FT) attribution metrics in a Program Opportunity Analysis report. The person that clicked the free trial button will need to be added to the Contact Roles section of the Salesforce Opportunity for the attribution to occur.