I found this helpful article on integrating CM with existing forms:
However, it doesn't let you capture custom fields, so I'm wondering how I can do that (ideally with a solution as simple as that above) - any ideas? Do I need to learn the API in order to do that?
Hi onesizeup, welcome to the forums! It's hard to say why custom fields aren't being captured in your specific scenario - would it be possible for you to contact our support team with details regarding your form, your account and the issues you're experiencing? We can get our dev team to recommend a suitable solution for you.