Include source PR description in the backport PR

I'm trying to amend the default backport PR description by including the source PR description, but cannot find a solution.

I've tried different variations of this setting, but no matter what I put here the backport PR description doesn't change. 

```json
  "prDescription": "{{defaultPrDescription}} \n\n Source PR\n\n{{sourcePullRequest}}",
```

Is there anything I'm missing?
