If you created a non-geo-tagged task (square icon) and would like to place that task directly on a plan (map pin icon), you are able to do so on the web. This ability is not available on the mobile app.
First, open the task and leave a new comment with the plan name using the @ sign ("@planname"). This will then associate the task with that particular plan, and you'll see the task icon change from a square to a diamond to signify this.
Next, open the plan you tied that task to and locate the task in the column along the right side of the screen. Right click on the task and select "Move on plan," and you'll see the task icon change again, this time to a map pin icon. This is because the task is now geo-tagged and will appear directly on the plan as a map pin, and you can move it to anywhere on the plan that you'd like.
You can view an example of this process in this video here: