Adding comments to task with API

I try follow the documentation with: (sanitised)
curl -H "Authorization: Bearer xxxxxxxxxxxxxxxxxxxxxxxx" https://app.liquidplanner.com/api/v1/workspaces/xxxxxx/tasks/xxxxxxxx/comments '{"comment":{"comment":"Written through API outside of LP - Hope your code is going well"}}' -X POST

returns with:
{"type":"Error","error":"BadArgument","message":"Expected a hash of attribute values for comment parameter"}curl: (3) nested brace in URL position 11:
'{comment:{comment:Written through API outside of LP - Hope your code is going well}}'

I've tried same without the single quotes as some have done and similar response, any ideas please?