To authenticate on the destination server, you need to use RequestParameters property of TElHTTPSClient class, which references an instance of TElHTTPRequestParams class. TElHTTPRequestParams class includes Username and Password properties, where you can specify username and password for the remote host.
If you need to use NTLM ("Windows Integrated") authentication, set UseNTLMAuth property of TElHTTPSClient class to True. Note that NTLM authentication is almost never used on public hosts. It's mostly used in Intranets. If you need to use Digest authentication, set UseDigestAuth property of TElHTTPSClient class to True.