Bitbucket Pipelines - Create Pull Request (PR) with default reviewers

bitbucket-pipelines.yml

image: node:12

definitions:
  steps:
    - step: &create-pr
        name: Create Pull Request
        caches:
          - node
        script:
          - apt-get update
          - apt-get -y install curl jq
          - '[[ "$BITBUCKET_BRANCH" == feature/* || $BITBUCKET_BRANCH == bugfix/* ]] && export DESTINATION_BRANCH="develop" || export DESTINATION_BRANCH="master"'
          - '[[ "$BITBUCKET_BRANCH" == "develop" ]] && export CLOSE_ME=false || export CLOSE_ME=true'
          - >
            export BB_TOKEN=$(curl -s -S -f -X POST -u "${BB_AUTH_STRING}" \
              https://bitbucket.org/site/oauth2/access_token \
              -d grant_type=client_credentials -d scopes="repository" | jq --raw-output '.access_token')
          - >
            export DEFAULT_REVIEWERS=$(curl https://api.bitbucket.org/2.0/repositories/${BITBUCKET_REPO_OWNER}/${BITBUCKET_REPO_SLUG}/default-reviewers \
              -s -S -f -X GET \
              -H "Authorization: Bearer ${BB_TOKEN}" | jq '.values' | jq 'map({uuid})' )
          - >
            curl https://api.bitbucket.org/2.0/repositories/${BITBUCKET_REPO_OWNER}/${BITBUCKET_REPO_SLUG}/pullrequests \
              -s -S -f -X POST \
              -H 'Content-Type: application/json' \
              -H "Authorization: Bearer ${BB_TOKEN}" \
              -d '{ 
                    "title": "Merge in '"${BITBUCKET_BRANCH}"'", 
                    "description": "Automated PR creation process :-)", 
                    "source": { 
                      "branch": { 
                        "name": "'"${BITBUCKET_BRANCH}"'"
                      }
                    },
                    "destination": { 
                      "branch": { 
                        "name": "'"${DESTINATION_BRANCH}"'"
                      } 
                    }, 
                    "close_source_branch": '"${CLOSE_ME}"',
                    "reviewers": '"${DEFAULT_REVIEWERS}"'
                  }'

pipelines:
  branches:
    '{bugfix/*,feature/*}':
      - step: *create-pr
    '{develop,release/*,hotfix/*}':
      - step: *create-pr

@artitambe26

What is bit bucket repo slug exactly?

Just passing through, but the official documentation for automatic environment variables like BITBUCKET_REPO_SLUG is here.