Use "Custom Object Record Services" (formerly called "Data Card Services).
Just go to Contacts-> Custom Objects and select the custom object set you want. Then from the "Custom Object" menu on the right, choose "Custom Object Record Services". Click "New Data" and add a processing step "Add to Step in Program Builder" and select the program to which you want to add these custom objects.
This will simply throw the object into this Program whenever a new object is created, which is presumably what happens when a prospect fills out your form, right?
If you need to throw them in a program when they're modified you can do that as well with a "Modified Data" service.