Flat surfaces require no pre-existing curves or geometry.
Use the Surface wizard or select Flat surface from the surface toolbar flyout.
Name the surface for future reference.
Enter the coordinates
for a corner point, or click the Pick point 
button and
pick the point with the mouse.
Enter the coordinates
for the diagonally opposite corner point, or click Pick point button and pick the point with the mouse.
Enter an (optional) elevation
offset for the surface. You can also pick the z coordinate of any point
using the Pick point button.
Click Finish or OK to create the surface.