Edit the CI Job, select "Triggered by git commit to branch", then click on the "Setup Webhook" link to open the details pop-up screen.

In the pop-up details, copy the URL. This will be used in the Bitbucket settings page later.

Now, log into Bitbucket, go to Repository settings > Webhooks page and click on "Add webhook":

In the "Add new webhook" page, enter the Title, and paste the value in the previous step into the URL field:

Click on "Save" and the webhook is set!