To add a new custom field for your campaign you first have to open the campaign that you want to edit. Them go to Customize Lead Fields menu on the right side of the screen.

Then scroll down and click on “Add More Lead Fields”. More options will open and there you have to enter the name of the field. You’ll have to choose the type of the field and to save it.
After that you can put the new field higher in the list with the fields so the agents will see it first. To do so just drag the field on the position that you want and save it.