There isn't a predefined "Are you sure" dialog, but you can roll your own.
Assuming you want a confirmation for a numeric entry, here's one method:
Set up an internal tag in the C-More. Have your primary numeric data entry object write to that tag. In the C-More's event manager, create an event that makes your confirmation screen pop up whenever the value in the internal tag changes. On the confirmation screen, have the "Yes" response copy the internal tag value to the PLC tag and the "No" response close the confirmation screen. To make the confirmation disappear after a "Yes" response, create an event that closes the screen when the internal tag value is equal to the PLC tag.
Note that this is off the top of my head. I haven't actually tested it, but it is how I would approach it.
