Here's where the parameters are being passed to N/task in my user event:

var mrTask = task.create({
                    taskType: task.TaskType.MAP_REDUCE,
                    scriptId: 'customscript_updatelocations_mr',
                    deploymentId: 'customdeploy_updatelocations_mr',
                    params: {
                        locations: locationsToUpdate,
                    }
                });
                mrTask.submit();