I know there is a Todoist pack. But what I am trying to wrap my head around is how to use it.
I would like to be able to click a button that launches a window or frame with for a new task that I could fill out. Our use variables from the doc to add to the new task.
I think your question is more about how to use Coda buttons in general than how to use the Todoist Pack specifically.
I recommend you first take a look at Overview of Buttons from Coda help center, after taking a look it will be super clear how to use buttons and the Pack to execute the action you want.