Read the document published by HCL on its support portal. This whitepaper explains in detail how to install Sametime 12.0.3 on Windows, run it on the same computer as the existing Domino server, and customize the user interface.
https://support.hcl-software.com/csm?id=kb_article&sysparm_article=KB0129431
The authors of this whitepaper are HCL Ambassador, Sametime expert, and my friend Roberto Boccadoro and me 😉
