Create a script that posts data to another domain

I have a website for a web-app that I’m hosting with Webflow. The app itself is hosted at a subdomain, which is located on our own VPS.

We’d like users to be able to sign up from the Webflow website. Is it possible to use some custom code that posts form input data to a PHP script that is located on a different server? With AJAX we seem to be unable to, due to a cross domain security restriction.

Does anyone have experience with this, and knows a way how to accomplish using AJAX for scripts on other domains?