I am trying to connect to a supplier API but they have a whitelist of the IPs which can consume their API. I gave them my server IP so that they could add it to the whitelist but it is still not working.

I suspect this is coming from my company proxy which hides IP, so I did some tests : weirdly enough, I noticed that the real IP of my server could not be detected on most websites like "who.is" or "whatismyipaddress.com". But as soon as I would try the same thing on websites using HTTP such as "http://www.mon-ip.com/en/my-ip/", then the proxy is detected and my server IP is found out.

I don't have a very deep understanding of HTTP and HTTPS so I have no idea how using http makes it possible to detect the proxy and find out the real IP. I tried checking request headers and it doesn't seem that the IP is transmitted in HTTP requests...

I've looked up some topics on SO and couldn't manage to find anything closely related to that matter (best I found was How can a website find my real IP address while I'm behind a proxy? which is somehow confusing me even more). Would someone be able to enlighten me on how http makes it possible to detect proxies and find out the real IP being used ?