zerotier-one runs in the background on a machine, and is interacted with via a REST api via the zerotier-cli command or the UI tray application. As it’s a rest API, an API key is generated on first start of ZeroTier and that’s what’s used to communicate with the backend. Assuming you’re using an administrator command prompt to use zerotier-cli, there’s nothing you need to do manually to make use of this API key. It’s done automatically.
The API key you generated at https://my.zerotier.com/ is not the same token for the same use. That’s a token for use against the API at https://my.zerotier.com and will not work with zerotier-cli