Create a node called Make PostProcess Settings. Click on it and find Post Process Materials in the Details tab. Tick the box to expose the pin on the node. Drag off the pin and promote it to a variable. In the variable (after compiling) add an array element and choose Assert Reference; select your material there.
Upon camera switch apply the Post Process Settings:
The Settings with the variable connected will add the material; if the pin is unconnected, no material will be used.
Don’t mind the Gain pin, that’s just my thing.