On the Settings tab you can enter the Key, its name, and a description.
You will also define the value type to be either free or predefined. If the value type is free, the values must be explicitly declared on the demand tag when applying Key-Value Targeting. If the value type is predefined, then the you will define them in the Values tab.
As long as your key's value type is set to predefined, you can add values via the Values tab. In the Values tab, you can either add values individually by clicking the +Value button or you can upload a CSV with values and names.
Like domain and app name lists, you can create value lists for your custom keys. To do this, go to the Values List tab of a key and click "+Value List". You can enter values manually or upload a csv. These lists can be used when a key is targeted on the demand tag.
Go to the Demand page to learn how to apply key-value targeting to demand tags.
Go to the Supply page to learn how to add keys and values to your supply tag for reporting.