An assembly can be attached before or after doing the takeoff. Click on the three dots next to the takeoff name to get to the sub-menu, and then click "Edit"
At the bottom of the screen will be an Items and Assemblies section ( this is also viewable when creating a takeoff). Click the green "+ Add items and assemblies" button
The folders contain the assemblies. Click to expand the folder and sub-folders. You can attach one assembly or multiple assemblies by clicking the green "+" to the right of the assembly. Then click the Add Assembly button, and answer the questions to yield the materials