Simply calls MessageBox using the Windows API and returns the result.
This is very useful if you want to set reminders through Task Scheduler, so much so that I don't understand why this isn't a standard app.
The syntax is "messagebox content title type" where type is an integer representing various properties of the message box. The full list of message box types and return codes are available in the Microsoft documentation for MessageBox here.
messagebox "This is the content of the message" "Title" 0
Since the result of the messagebox is returned, its value is stored in the errorlevel variable.
start /wait messagebox Continue? Confirm 4
echo %errorlevel%