In some situations you might want to prefill some fields for the user. This can be achieved by the following method. 
JSONObject prefill = new JSONObject();
try{
    prefill.put("description", "Prefill the description with this.");
}catch (Exception ex){
    ex.getStackTrace();
}

PrefillHelper.getInstancen().setPrefillData(prefill);
You need to pass a key value pair to the setPrefillData. The key is the form identifier, which can be found under the ‘Advanced’ section of a question. (see screenshot for an example) Form identifier