You can use a bitmap file or procedural map to control the Metalness material component. White pixels in the map increase metalness. Black pixels reduce metalness to 0. Intermediate values adjust metalness accordingly.
Using a Metalness map
Top: No map
Bottom: Using a Noise map as the Metalness map
The metalness parameter is available with the Strauss shader.
Reducing the Amount of the metalness map reduces the map's effect, and increases the effect of the Metalness value on the Basic Parameters rollout. When the Amount is 0 percent, the map isn't used at all.
3ds Max opens the Material/Map Browser.
(If you choose Bitmap as the map type, 3ds Max opens a file dialog that lets you choose the image file.)
Alternatively, you can use the Slate Material Editor to wire a map node to the Metalness component.