将表单数据从外部应用程序发布到Rails后端

For marketing reasons we want to a/b test some landing pages. Fairly typical but, ideally, we'd like the page to post directly into our Rails backend (creating a new user). We plan to host our landing pages on Unbounce (or whatever, doesn't really matter) but it's not clear how to post to (For marketing reasons we want to a/b test some