The custom complication was added in the latest Galaxy Watch Designer update (version 1.8.1 beta). How is it used? Nothing happens when I click on it.
You have to insert some components into design, then select and save them as custom complication, before you can insert it as complication again.
Its mentioned on the bottom of tutorial page developer.samsung.com/galaxy-watch/design/watch-designer/gear-watch-dev-complication
If you need to re use some components you can save as “Custom” Complication. It will listed in complication items. For EX: if you re use Hour, Minute and seconds hand >> select this layers and save as custom complications and same set listed in the custom complications. Its very useful option available in latest version.