To create a task from a message, hover over the message until the "..." icon appears, or press and hold the message on your mobile device. From there, choose "Convert to task". If you don't see this option, please ensure that you have enabled the task creation option for the chat.
Once a message becomes a task, you'll see it appear as a button with a circle where you can check the task as done. To open the task, simply click it.
To mark the task as done, click the circle.