@chrisdarroch Sadly this is indeed a limitation in the @Bitbucket API. They already have a ticket in their issue tracker discussing this, but it seems to be going a little stale. Do you need to keep your repo in Bitbucket? Both @github and @gitlab support that feature.
https://bitbucket.org/site/master/issues/5814/refify-pull-requests-by-making-them-a-ref
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