To extend a linear form to an imaginary plane

Select (Modify Form).
Click Extend a Linear Form icon.
From the Method option menu, select To Plane (3 Pts.).
Select a linear form to extend.
Enter 3 points to define a plane. The form extends along its baseline and projects against the plane.
Note: Many linear forms have baselines located on a face other than that which seems to be the base of the form. To verify which face of your form is the baseline, select the Show Form Orientation tool. The resulting triangle, displayed along the base of the form, points in the direction of the base plane.

Note: The form will only extend along the plane of form orientation. The form orientation indicator lies in the direction of the baseline.