Synergy is an incredibly useful keyboard and mouse sharing utility I have used for years.
https://symless.com/synergy

I've never had an issue connecting a Windows PC to a OSX desktop.

However, now that my desktop is Ubuntu I have had zero luck with Synergy to connect to my Windows laptop.

I've tried setting Synergy in Ubuntu to both Server and client and tried every possible permutation in terms of configuration settings.
No luck.

Any chance someone reading this has successfully used Synergy in Ubuntu? If so, is there some trick or hidden feature to make this work?

A search of synergy and ubuntu generates a pretty good amount of hits, including version 18.04 of Ubuntu along with synergy 2.0.10. From all that I see, several users have recommended clean installs, and then another user worked towards a more definitive process which would not require clean installs, but ensure that the prerequisites were present in the system. That hit was on symless's forums page.

There does not appear to be any hidden features to get it to work.

The Ubuntu community also has a pretty extensive guide, but it notes that it requires updating for the latest version of Ubuntu. For that I'd refer to that first hit on Symless' forums site.