You can automatically add a task list to a ticket at any point, based on any conditions during its lifecycle.
The steps to do this are:
1. Switch the task list to automatic
To switch automatic functionality on for a task list simple go into the tasks admin screen and click on the desired task list.
Then click the edit button at the top next to the task list name.
Finally, click the switch to turn automatic on and click 'OK'.
2. Create a trigger/automation that adds the task list
At the point you set a task list to be automatic, a Zendesk target is automatically created. You can use a Zendesk trigger or automation to fire this target, which will add the list to the ticket.
To create a Zendesk trigger click on the admin cog on the left of the screen and then under triggers, click on 'add trigger'.
Then give it a name. Next, under "Meet ALL of the following conditions" you will need to define the conditions under which the task list will be added to the trigger. For example, you might want a list to be added at the point the ticket is created, and the form is X. Or you might want to be more specific and only have the task list added based on a drop-down field selection, or when a specific tag is added to the ticket... it's completely up to you!
IMPORTANT NOTE: Whatever you set your rules to, make sure that after the trigger fires that the trigger or automation no longer meets these conditions the next time the ticket is updated. This is called a nullifying condition. Read more about nullifying conditions here.
For this example, we'll be adding a task list to a ticket when it is created and a specific form is selected like this:
Now, all we need to do is, under 'ACTIONS' add the rule: "Notify target".
You'll then need to look for the name of your task list... it will look something like this:
At that point, you will be able to click "Create" at the bottom right.
Note, before clicking create you will probably need to click the X next to the URL parameters as they're not required. At that point, the create button will become clickable.
To test out adding the task list, simply create, or go to a ticket and replicate the conditions the trigger needs to fire.
That's it! You're done.