Merge through Pullrequest API not using JSON parameters

I’m attempting to merge via the pullrequest API.

curl -X POST -k -H 'Content-Type: application/json; charset=utf-8' -H 'Authorization: Basic ENCODED_STRING' -i '' --data '{
  "pullrequest_merge_parameters" : {
  "type": "pullrequest",
  "message": "Automated Commit for branch sync",
  "close_source_branch": false,
  "merge_strategy" : "merge_commit"

I get a response:

HTTP/2 200
server: nginx
vary: Authorization
vary: Accept-Encoding
content-type: application/json; charset=utf-8
strict-transport-security: max-age=31536000; includeSubDomains; preload
date: Mon, 02 Dec 2019 17:54:24 GMT
x-served-by: app-1119
x-static-version: 17f8660d50c1
x-content-type-options: nosniff
x-accepted-oauth-scopes: pullrequest:write
x-credential-type: password
x-render-time: 1.49699115753
x-request-count: 1761
x-version: 17f8660d50c1
x-frame-options: SAMEORIGIN
content-length: 6621

and I get a merge.
Unfortunately, the pullrequest API doesn’t seem to use the JSON parameters. The “message” isn’t used and neither is the “merge_strategy”.

Any help would be greatly appreciated.