How to allow screensaver while fullscreen app is active

Wondering how do I allow a timed screensaver to activate while a fullscreen app is active. For example, a YouTube video is playing fullscreen which normally would prevent the screensaver from activating. How do I get the screensaver to remain active?

Running Windows 10 Pro

This is controlled by the program. Programs call SetThreadExecutionState to tell windows not to sleep or turn monitor off while playing video then calls it when the video ends to go back to normal. See https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-setthreadexecutionstate.

So you'll find the setting, if a program has one, in the program's settings. EG in Windows Media Player Options it's Allow Screensaver During Playback.