You can set auto_merge: true option to automatically merge the PR when all checks pass (including successful apply). Note that it’s a top-level setting, not project level: 
projects:
- name: development
  dir: dev
- name: production
  dir: prod
auto_merge: true
auto_merge_strategy: "squash" # optional, only supported on github
If you are using github VCS you can also specify auto_merge_strategy to define the type of strategy. Possible values are:
  • squash: for squash merge
  • rebase: for rebase merge
  • merge: for merge commits merge