On the server config page, check "Use existing configuration" instead of "Configure interactively". You will then need to create a configuration file, which, if I recall correctly, Synergy will give an *.sgc extension. Feel free to use mine as a starting point, see below. For reference, my screens are arranged as:
Fractal Dell_laptop
surface-pro-3
Hope that helps.
section: screens
Fractal:
halfDuplexCapsLock = false
halfDuplexNumLock = false
halfDuplexScrollLock = false
xtestIsXineramaUnaware = false
switchCorners = none +bottom-