Request blocked by CORS policy

Hello.
could you please help me. I don’t understand why my POST request doesn’t work. I make the post request from frontend as a formData. What kind of endpoint should be? I see you make https://numerical-methods-solver.onrender.com/api/v1. But if i have an index.php in the root of my app, how can i receive back the response to frontend. The post request with the endpoint like **/v1/index.php or **/api/v1/index.php doesn’t work.
This is my question here: POST request is wrong
Thank you!