//create the CompleteTaskModel structure and fill it in 
var model = new CompleteTaskModel();
//fill in the "notify coexecutors" field
model.alertCoexecuters = true;
//fill in the "Start date" field
model.startDate = DateTime.Now;
//fill in the "Time report" field (in minutes)
model.worklogMinutes = 120;
//create ICommentActionModel 
var commentModel = InterfaceActivator.Create<ICommentActionModel>();
//create object IComment 
var comment = InterfaceActivator.Create<IComment>();
//fill in the "Comment text" field
comment.Text = "Comment";
//fill in the "Creation date" field
comment.CreationDate = DateTime.Now;
//fill in the "Comment author" field
comment.CreationAuthor = PublicAPI.Portal.Security.User.GetCurrentUser();
//сохраняем comment
comment.Save();
//fill in the "Comment" field
commentModel.Comment = comment;
//fill in the CommentActionModel
model.actionModel = commentModel;

//complete the task
PublicAPI.Portal.TaskBase.Task.CompleteTask(taskId, model);