An extension can change any line in the request header.
Example code:
--[[ <HCExtension>
@name Change User-Agent in the request headers
@event RequestHeaderReceived/Request
</HCExtension> ]]
function Request()
hc.request_header = re.replace(hc.request_header, [[^User-Agent: \K[^\r\n]+]], 'My_Agent')
end