Generally PBR materials don't use a Specular map in the same way a legacy material does, but if you are looking to create a PBR material using legacy style textures the best place to put it would be in the Glossy Layered Weight slot. "This controls the glossy properties for the base layer of the shader."

Here is a good reference point for what each setting does on a PBR Specular glossy Uber shader does in DS.

Iray Uber Shader Overview

There are some good general PBR guides available here also:

https://www.allegorithmic.com/pbr-guide