Disable Custom Navigation for Forced Video Viewing

Good Morning!

I am working on a web-based training using Captivate 9 (2017) that links to videos in another location - e.g. not embedded within the WBT.  The desire is to have the forward navigation (using custom navigation buttons) be disabled until the video has been viewed (or at least in part - roughly half).  The disabled navigation should only happen the first time the slide is viewed.  If the user returns to the slide, navigation should be available without being forced to wait or watch the video.  I am struggling to create the Conditional Action.

Any insights on what variables and conditional action script?  I'm really drawing a blank. Any thoughts would be greatly appreciated.

Thanks!