The Character ID is the long string after the /c/ in a Character Link.
For example, for Text Adventure Game the Character Link is https://beta.character.ai/c/M5xMXf4FKepKTYtWPqVaEZzuEuy90uu0eNZr4GZtDsA and the Character ID is M5xMXf4FKepKTYtWPqVaEZzuEuy90uu0eNZr4GZtDsA
You can copy a link to a Character by clicking Share on the top right of the chat window.