Setting user-agent

Consider a scenario where post request is completed successfully for a request with specific user-agent . I want to create a html poc for the same with the respective user-agent. If it is possible how can it be done.

I read that user-agent is not in the forbidden list as per mozilla documentation : https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name . So I hope it can be achieved

Why almost all free HTTP Proxy server doesn’t change the User-Agent?

I’ve created a automated test suite for proxy server. From several websites in the internet like https://free-proxy-list.net/ I get many IPs of free proxy servers. I’ve also created a own Website with JSON API to get HTTP requests header.

All requests mostly differ in values like X-Forwarded-For, Via and X-Proxy-Id. But no proxy server changes the User-Agent. Its always the User-Agent from the origin, in my case python-requests/2.20.0.

After researching I know it’s technical no problem to do this with a proxy but anyone now why no free http proxy server do it?