How could I get the web browser's proxy infomation like proxy server adress and port?

I have a software run in the client's device, and I need to know the proxy info for keeping in the same internet. some web browsers could configure web proxy for its own like firefox. Thks!

I think if a web browser has used a web proxy, every request of it may be sent to the proxy server, and I can know it from the request headers but I don't know how to get that request headers info in an outer software.

and I have seen API that could get the proxy info in browser extensions dev doc. but could I call that API in outer software?

and maybe the web proxy info is set in an absolute path, but I don't know where is it.

get the web browser' web proxy info