I just went through something similar on an Uberflip integration. Dan's Click Link trigger will work great for known visitors. However, if someone is not known, they won't get added to the Program because they are anonymous.
Another option to consider is using the Filled Out Form (assuming that's the activity you want to track and the API you are using allows you to use this trigger). Or you use in combination with
Lead is Created trigger. You'd then use the
Has Visited Web Page filter to specify your page. The important things is to add the timeframe. As long as the person visited that page in the last x minutes, they would be added to your program. You could also try the Clicked Link filter.
Does this make sense?