Create an assignment activity configured to allow only to allow only text (Submission types field set to Online text) Just under the previous setting, set the field Word limit to 1 As admin, enable "Mobile services": Site administration ► Mobile app ► Mobile settings Create a Token in the mobile app service for one student in the course: Click on Site administration ► Plugins ► Web services ► Manage tokens Next, you can do a CURL REST call simulating a WS client with the user. You need to replace the wstoken, assignmentid with the assignment activity instance id and the URL of your moodle instance curl 'http://localhost/m/stable_master/webservice/rest/server.php?moodlewsrestformat=json' --data 'assignmentid=37&plugindata%5Bonlinetext_editor%5D%5Btext%5D=%3Cp%3Easdf%20asdf%3C%2Fp%3E%0A&plugindata%5Bonlinetext_editor%5D%5Bformat%5D=1&plugindata%5Bonlinetext_editor%5D%5Bitemid%5D=0&moodlewssettingfilter=true&moodlewssettingfileurl=true&wsfunction=mod_assign_save_submission&wstoken=12a2a65aeea2a177055e233f9e303218' --compressed | python -m "json.tool" # Check that as curl response you receive a valid json. The first element of the json response array contains the field "warningcode" set to couldnotsavesubmission, and the "item" field containing a text with HTML characters encoded.