xseturgent is a tool that sets/clears the urgency hint of a window in X11

xseturgent -h
xseturgent -V
xseturgent [-i <windowid>] [-t <set/reset/toggle>]

-h Help.

-V Version information.

-v Verbose, show files changed.

-i windowid The window ID, in decimal or hex. If not provided, use the WINDOWID environment variable by default; fails if that doesn't exist.

-t <set/reset/toggle> What to do with the urgency hint.