Hi Vishal, thanks for your question!
If you’re trying to sync users from another domain, try using your existing Authentication Proxy to perform that sync (if the Authentication Proxy can communicate with the other domain where it resides now). See this Knowledge Base article for more information: Can I sync multiple directories with one Duo Authentication Proxy server using multiple [cloud] sections?
Since you didn’t mention in your post which application you’re referencing and how it authenticates now, try using this other Knowledge Base article if primary authentication is currently using the Authentication Proxy: Does the Duo Authentication Proxy support authentication against multiple Active Directory domains using a single [ad_client] configuration?
Ultimately, I recommend contacting Duo Support since this is an environment-specific and application-specific question. They’ll be able to provide more targeted support. Hope this helps, and thanks again for reaching out!