Open the Effects tab to gain access to the Post Crop Vignetting sliders. Using them will darken the bright edges and corners of the image.
Get Familiar With The Sliders:
Note the hard edge of the circle. It corresponds to the Feather slider, which is set to zero. Adjusting the slider softens the edges of the darkened area. Experiment by moving it left and right and watch the changes in real time. All the way to the left creates a hard edge. All the way to the right creates the softest possible transition.
With the Feather slider still set to zero, experiment moving the Roundness slider to the left and right, and watch the shape of the vignette change. Depending on the subject, use the shape that best fits the image. At +100, a circle is formed. At zero, an oval is formed. The more it’s moved to the left, the shape becomes rectangular or square.
Experiment moving the Midpoint slider to change the overall size of the vignette. The size should be dictated by how much outside darkening you need to create.
Once you feel good with the overall shape/size/amount, adjust the Feather slider to smooth out the transition. Fine-tune each slider to finalize the effect.