I've got the following problem: in Sitefinity (9.1, Feather) I need a form, which can call 3rd party API (Mandrill) once submitted.
As far as I understand, I need some kind of custom widget or something.
Any help would be very appreciated.
Thanks
I would start here. You don't necessarily have to create a separate class library for creating the custom Feather widget (you can just put it in the SitefinityWebApp web project), but you can if you like. With Feather/MVC widgets you basically get a Controller and View, with an optional Model class to play with too.
In your scenario, you'd probably have a Controller with two actions: Display the form, and handling the form submission. In your form submit action you'd then call Mandrill to submit the data (or do whatever it is you need to do). In your Controller you're in C# purely, so you can do whatever you like there.