Matt,
I think the best course of action right now is to make this read only. I'll explain why.
The related field represents the activity number that this activity is related to. If the current activity was created through the schedule a follow-up checkbox, you wouldn't have to search for it using the dropdown.
In the event that you're creating a new activity and want to relate it with an old activity, on the left side of the screen you'll notice a grid for activities. This grid filters to show only activities for the contact you are creating the activity for. If you would like to relate this activity with a previous activity, you can do so by clicking the "relate" link next to the activity in the grid on the left.
Normally for follow-ups you don't have to do this because this field gets updated automatically when you create the follow-up.