To add an item there you would need to create a Bitbucket Connect App.
This location in particular is a WebItem at the location org.bitbucket.pullrequest.summary.actions. With this WebItem you can open up a new page or open up a dialog box whose contents are an iframe.
Learn more here: https://developer.atlassian.com/cloud/bitbucket/integrating-with-bitbucket-cloud/
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