Go to Tools -> Customize... -> Toolbars -> Add...

Select "Format" in the Category list.

One of the items in the Commands list is "Flash". When placed on a toolbar and
activated, this applies the effect instead called "Blinking" in the "Font
effects" tab of the Character dialog box in in all other places and Help files
that mention this effect.

The word "Flash" should be changed to "Blinking" for consistancy, to make the
purpose of this toobar button immediately apparent to a user selecting toolbar
buttons. (The user can change the text from "Flash" to "Blinking" or to anything
else after placing the item on a toolbar, but the default should be the name
used elswhere in Writer and the help files.)