Overview & background
You can associate a task with a plan by adding a plan to the "Task Attributes" section or you can leave a new comment with the plan name using the "@" sign ("@planname").
Using the 'tagging feature' to associate the task with a plan
Drag and drop
You can drag and drop a task onto a plan. To add a task to a particular location, on the web, you can simply drag-and-drop it onto the plan.
Open the plan in question and locate the task in the column along the ride side of the screen. Simply drag the task onto the plan where you would like it, and you'll see the task icon change again, this time to a pin icon. This is because the task is now geo-tagged and will appear where you place it.
Note: This ability is not available on the mobile app.
You can even drag-and-drop multiple tasks at once like this:
Video Tutorial on Creating Tasks
Video Tutorial on Advanced Task Features