How to add Occlusion Map in Surface shader

I want to know that how we can add occlusion map in writing surface shader.
Like we do
properties{
_maintex(“MainTexture”, 2D) = “white” {}

_OcclusionMap (“OcclusionMap”, 2D) = “?”{}
}

This page says default is 1, so probably your default texture should be white as well.