This is not a hybrid deployment scenario. Lync online and exchange on-premise are isolated domains. I have deployed on-premise exchange server with SMTP domain called ‘contoso.com’ and Office 365 Trial domain is Lynctest12.onmicrosoft.com’.
I have configured outlook and Lync-online on the same client machine. Downloaded and installed outlook conference Add-in. When I tried to schedule a Lync-online meeting, I got the meet URL with contoso name space and that doesn’t exists. (Fig.1)
Later, I have added Lync-online SIP URI as the new SIP proxy address on the exchange custom email address tab and saved the changes (Fig .2).
Also, opened ADSI edit container and selected the on-premise mailbox, changed mail attribute to Lync-online SIP URI and saved the settings. (Fig.3)
I have signed out of Lync-online and outlook and came back. I was still using same mailbox (firstname.lastname@example.org ) . When I clicked on new Lync online meeting, I got new sets of meet URLs and conference details.These URLs were pointed to office-365 Lync online. (Fig.4)
I have tested above meet URL using Lycn web-app and I was able to login successfully. (Fig.5)
I hope above information would be helpful when you have on-premise exchange server with Lync-online.