When a router config is saved with

copy run start

IOS asks for a destination filename:

RTR#copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]

To bypass the request and use the default name startup-config do this:

RTR(config)#file prompt quiet

And then again

copy run start

will not require the filename:

RTR#copy run start
Building configuration...
[OK]

This command may be useful inside EEM scripts that can’t run intecartive commands.