I want to POST data using Laravel API but it can only be accessible using VPN

I'm developing a Website that posts data to another API and returns its response.

Http::post('API_Url', [ "Key"=> ""]);

I am using Http Client to Post data but it returns me the error "403 Forbidden". How can I post data to API and get its response? when I use VPN and test this API using POSTMAN it gives me exact response that I want...