The most convenient way to create a cookie for use with the Built-in Render Pipeline is to create a grayscale texture, import that texture to Unity, and then let Unity convert the texture’s brightness to alpha.
Note that in the Built-in Render Pipeline, cookies only use data from the alpha channel. This means that you can define a shape for a cookie, but not a color.
To do this:
Note that the pixels of a cookie does not need to be completely transparent or opaque, but can also incorporate any values in between. You can use in-between values to simulate dust or dirt in the path of the light, or to simulate caustic effects such as those produced by the ridges in a car’s headlight.
For more information on configuring the import settings for cookies in the Built-in Render Pipeline, see Texture Type: Cookie.