How do I use an Ambient Occlusion map with the Principled Shader? There are fields to use PBR maps for Base Color, Normal map, etc, but I don't see one for AO. Or is there a way to multiply the AO map with he Base Color map? If so, how?

I have tried using a Layer Mix node but this blends the maps making the Base Color map less saturated, so it is not the way.

You don't use an ao map with a photorealistic material. The lighting is calculated using path tracing.

There is a "passthrough" slot on some baking materials that tell the exporter which map will be the ao map on export. The principled material might not have this passthrough slot.

I have a reason for using the AO map. It is often used in game engines. In Maya, for example, you can directly feed it in to the material.