To connect the top of a form to an imaginary line

Select (Modify Form).
Click Modify the Height of a Form icon.
From the Method option menu, select To Imaginary Line..
Enter two data points to define the slope of the line to connect to. An imaginary plane is generated through the two points, perpendicular to the view in which the first point is entered.
Select a form to connect with the line.
Accept or reject the connection.

Front view of original form.

Front view of the form after connecting the top line and plane.