To add a new Deployment Key in Codebase, first navigate to your repository's Settings to the right hand side of the browser:
Now go to the Deployment Keys tab, and click Add new SSH public key:
Now enter a name for your new key, paste the key into the Public Key field, then click Add Public Key to save it.
Now click the Back to repository button to the top right of the UI to go back to the repository browser, then copy the Git URL found in that same area on the browser:
You can now see further information about how to update the repository configuration in Deploy in this article.