To add a new Deployment Key to Bitbucket, navigate firstly to it's Settings via the link at the bottom of the sidebar:
Next, head to the Deployment Keys page:
Then click the Add Key button:
Now give your new key a description, paste the key in, and click Add Key to finish.
You'll now just need to head back to your repository's overview, then copy the SSH URL from the top right of the page:
You can now see further information about how to update the repository configuration in Deploy in this article.