Before you can add your webhook to Bitbucket, you'll need to retrieve it from your DeployHQ project.
Open your repository up in BitBucket (there's a shortcut at the top of DeployHQ labelled "Browse Repository") and click Settings, followed by Webhooks in the adjacent menu. Then, click Add webhook:
Enter a meaningful name for your Webhook, then paste the URL from your server in DeployHQ into the box into the URL field. Ensure that Status is set to Active, keep Repository Push selected next to Triggers and hit Save to finish.