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First, you need to edit your build's configuration settings. Go to the build's page and click Edit Configuration Settings in the top left right corner.
There choose the Build Features tab, where you'll be able to add Commit Status Publisher as a new build feature.
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Choose Bitbucket Server (Atlassian Stash) as a publisher, set the Server Base URL, and enter your credentials. Then just test the connection, click on Save, and you are all done!