When a task is mark completed a personal note will be added

When a task is mark completed a personal note will be added

Hi! This is what im trying to do:

After I mark a task as completed, i want a form to popup, say... "Task Result" and the user will fill up and submit.
Afterwards, the task result submitted will be added to the notes.

I tried it with custom workflow and Layout rules, but did not succeed.

Is there a way I can do this? How?
Thanks so much