Hi James,
You need to add a page parameter/dataview to your popup-window with the UserTask object,
from there just a button with Complete User Task click action;
https://docs.mendix.com/refguide/on-click-event/#complete-task
OR Within a Microflow → Complete User Task activity → https://docs.mendix.com/refguide/complete-task/ )
Best regards
With your current setup, you would just need to pass the object into the new popup page, and use a dataview on that popup page to accept the object. Then the proceed button will be able to do what you need.
You also might consider using the “Show Confirmation” action in nanoflow commons. It’s capable of directly prompting the user for a response and then continuing the same nanoflow based on the response.