Create pull request via API: destination repository cannot be changed?

Hello,

I'm trying to create a PR via the API, where the source is my own fork, the the destination is the main repository. 

I get this error:

{"type": "error", "error": {"fields": {"destination": ["destination repository cannot be changed"]}, "message": "destination: destination repository cannot be changed"}}

 Here's my command:

curl \    
  -X POST \
  -H "Content-Type: application/json" \
  -u username:app_password\
    https://bitbucket.org/api/2.0/repositories/username/repo-fork/pullrequests \
  -d '{      
  "title": "test",      
  "description": "testing automated PR",      
  "source": {        
    "branch": {          
      "name": "test"        
    },        
    "repository": {          
      "full_name": "username/repo-fork"        
    }      
  },      
  "destination": {        
    "branch": {         
       "name": "master"        
    },        
    "repository": {          
      "full_name": "username2/repo"        
    }       
  },       
  "reviewers": [ { "username": "user" } ],       
  "close_source_branch": false    
}'

Why does this editor remove all my newline breaks when I paste code from my editor? Anyway... any ideas how do I make a PR from a fork to the main repository?

Thank you!