Main toolbar > Place Highlight (on Align flyout)
Tools menu > Place Highlight
Keyboard > CTRL+H
The Place Highlight button, available from the Align flyout, enables you to align a light or object to another object so that its highlight or reflection can be precisely positioned.
From this flyout you can also choose Align, Normal Align, Align to Camera, or Align to View.
In Place Highlight mode, you can click and drag the mouse around in any viewport. Place Highlight is a viewport-dependent function. As you drag the mouse in the scene, a ray is shot from the mouse cursor into the scene. If it hits a surface, you see the surface normal at that point on the surface.
When you hit a surface, any selected objects are positioned along a line that represents the ray reflected off the surface about the surface normal. The objects are positioned along this line based on their original distance from the surface point. For example, if the object is 100 units from the surface point before being moved, it will be positioned 100 units from the surface point along the reflected ray.
If the object is a light, the position of the highlight on the surface of the object will be the surface point that you've chosen.
Tip: Place Highlight works with any kind of selected object. It can be used to move objects based on a combination of face normals and initial distance from the face. You can also use Place Highlight with a selection set that contains more than one object. All objects maintain their initial distance from the face. In this case it has nothing to do with highlights, but is simply being used to position objects.
Procedure
To position a light to highlight a face:
Make sure the viewport you plan to use is active, and that the object you want to highlight is visible in it.
The result of Place Highlight depends on what is visible in the viewport.
Select a light object.
Click Place Highlight or choose Tools > Place Highlight.
If the button is not visible on the toolbar, choose it from the Align flyout.
Drag over the object to place the highlight.
When you place an omni, free spot, or directional light, the software displays a face normal for the face the mouse indicates.
When you place a target spotlight, the software displays the light's target and the base of its cone.
Release the mouse when the normal or target display indicates the face you want to highlight.
The light now has a new position and orientation. You can see the highlight illumination in shaded viewports that show the face you chose.