menglim
We have different options for this such as installing the HTTPassthru Plugin (this requires additional external setup with something like kerberos, etc. to be able to send user/agent auth details via the webserver), we have the LDAP Plugin which allows for AD integration, and we have the OAuth2 SSO Plugin which allows you to integrate with any of your OAuth2 Authentication providers.
So if I understand you correctly you want people to use their company credentials (username/email + password) to login to osTicket instead of them having to create a new account specifically for osTicket with a separate password, right? If so, the options above will handle that. If they login using HTTPassthru, LDAP, or OAuth2 SSO the system will either link that to their local osTicket account or automatically register a new account for them.
Cheers.