See this thread, especially the last response in it.
First, Automic AE does not play well with UTF-8 (sadly), and I don't think you can use UTF-8 for the REST API.
Second, with some tools such as curl, you need to make an explicit conversion from UTF-8 to ISO-8859-1 before sending your REST URL.
If you wish to have the sorry state of affairs regarding AE and UTF-8 addressed in general, please vote for the associated idea.
Hth,