Proper way to secure API route?

[esteban89]

I have a form in my homepage than when submitted it adds data to a database.
I have it coded so that it sends a fetch with POST to /API.
It works perfectly, however I'm worried anyone could use for example Postman and add a lot of records to my database.

What's the proper and easier way to secure my API so that it can only get requests from my website?