The Character ID is the long string after the /c/ in a Character Link.
For example, for the Character Ira, her link is:
https://character.ai/chat/jgoe8pz4X_VYeqMJQp0a4v2dUUQgpRKoCJXq7cuHRhM
and the Character ID is jgoe8pz4X_VYeqMJQp0a4v2dUUQgpRKoCJXq7cuHRhM
You can copy a link to a Character by clicking Share on the top right of the chat window.