First, add an iFrame component to the Pivot Interactives activity.
When editing an activity, click Add Component and then select iFrame from the drop-down menu.
To add a Desmos graph, simply copy+paste the graph URL into the box labeled URL within the iFrame component.
The graph will appear as if you'd opened it in another browser window.
To add a Desmos activity, add this URL into the box labeled URL within the iFrame component: https://student.desmos.com/
When students access the activity, they'll be prompted to input the Desmos activity code for your activity. Once they've done that, they'll be taken to that activity.
Alternatively, you can input the URL with the code pre-populated.