CURLMBS setup error?

A custom office desktop app now has a connection to an external accounting software which uses a desktop app to offer a REST Api using JSONItems to communicate.

I use CurlMBS to talk to it, and while it looked like everything worked, we now encounter issues when strings containing umlauts or other special characters are being sent. The API is UTF8 capable (fun fact: It’s built in Xojo), and I checked that the JSON string curl sends is UTF8 encoded too.
When I copy the JSON string from the debugger and insert it into the built-in test client, everything works as expected.

Do I miss a Curl option to set up text encoding correctly, or could it be anything else related to curl?
Sadly the API only returns a “Parameter error” with no more further information. And without special characters the curl command works from inside my app.

And what do you do?

e.g. do you pas a content type?

and do you do ConvertEncoding when you assign to OptionPostfields to make sure it is UTF-8?

Thank you, Christian.
Turned out it was a setup error, but none of the kind of an unknown feature. I set the post length to JSON.toString.Length instead of .Bytes, which cut a closing bracket I only realised when I set the JSON to non-compact …

Great. Congratulations for finding it yourself!