How do I POST form data with UTF-8 encoding by using curl?

I would like to POST (send) some form data to a webserver using cURL on a terminal-prompt.

This is what I got so far:

curl --data-ascii "content=derinhält&date=asdf" http://myserverurl.com/api/v1/somemethod

The problem is that the umlaute ("äöü") are replaced by "?" when I receive the post request on the server.

I think I need to use an UTF-8 encoding for the POST request.

Does anybody know how I can achieve this?