Critical Javascript Question For My Product

Hi, I have a product idea. It works fine on my website. But when I add it to another website, it causes problems. I can't send requests from the 2nd website to my website.

Under my 2nd website
<script src="site.com/index.js"></script>
but it didn't work.

There is a PHP post mechanism in index.js. It updates data by making posts.

But it didn't work.

Is there a stable way to solve this? Can it be solved without CORS?