Not sure what your exact scenario is, so I’ll try to do my best here. There is a backend API you can use for creating a new form submission. To use it, you need to pass the data from the front-end form to the backend, which means you need to create a layer that fits your scenario. For example, this could be a custom controller acting as a REST API endpoint that accepts the form data and hands it over to the backend logic.
If you’re looking for a REST API already built into the product that directly submits form data, I doubt there is anything like it. The Form widget does expose an endpoint like:
/Kentico.Components/en-US/Kentico.FormWidget/KenticoFormWidget/FormSubmit?formName=<form-name>&…
However, this endpoint is antiforgery / CSRF protected for good reasons, so you cannot call it directly on its own.