In Preno you can set up Extras by going to Settings > Extras and adding them there. However you can also create a new extra directly from within a booking.
To create a new extra:
Click into the booking.
Scroll down to Extras. If it is a group booking, click on the room you would like to add the extra to first and then scroll down to Extras.
Click Add extra.
Enter the Name, Amount and Quantity .
Tick Bill to agent (not guests) if the extra is charged to the agent that made this booking.
The extra will not be saved for future use. If you would like to create an Extra that can be searched for and used on different bookings, create the extra in the Extras page in your Preno Settings.