Prebuild step with MSBuild

I have a powershell script that I would like to call as a prebuild step. I have managed to add a new External tool where I call my powershell script. In this script I call MSBuild which needs two arguments - platform (x86 or x64) and configuration (Debug, Release, ..). The problem is, those two parameters are part of the toolchain in CLion and I have no idea how to get those parameters from CLion to pass them to the powershell script.

Any help is appreciated!