How to get pull request key, source and target branch in Bamboo 6.6.3?

When a PR is created in Bitbucket Bamboo creates a build. In version 6.6.3 the following variables are not available

bamboo.repository.pr.key
bamboo.repository.pr.sourceBranch
bamboo.repository.pr.targetBranch

I need these values in order to pass them to Sonar for PR analysis. What is the workaround to get the above values?