How to programmatically create a bookmark for another connections user?

Question

We want to create bookmarks programmatically and this works fine if the email parameter matches the account of the currently authenticated user.

The question is, how can we create a bookmark for another user ? Below is the pseudo we use from the SBT.

String apiPath = "/dogear/api/app";
Response<String> postResponse = restClient.doPost(apiPath).parameter("email","another users email").body(body, "application/atom+xml").asString();

The log files are not very helpful, in the client we get an ERROR 400 Bad Request