Collective2 provides two kinds of API key, version 3 and version 4.
If you need the version 3 API Key to use for PlatformTransmit program, go here https://collective2.com/platform-transmit/manage/. The key will be automatically generated.
If you want to build sites/software based on Collective2 technology, you need the version 4 Developer API key. To create one, go here https://collective2.com/account-info. Click Configure APIv4 keys.