There should be a short guide in <Warlord 3>/Editors.
The code should uniquely identify your scenario.
Or in the words of the guide:
Click in the Code field. The code is the file name that will identify the scenario. It can be up to eight letters long. We are going to create a scenario based in the exciting world of Twodimensia so type in the code Twodimen. Now click in the Name field. Type in Twodimensia. This is the name that will appear in the scenario list at the start of the game. If you want everyone to know who created this wonderful scenario, enter your name in the Author field.