Add $(Configuration) and $(Platform) to External Tools arguments list
https://docs.microsoft.com/en-us/visualstudio/ide/managing-external-tools describes the currently supported arguments that can inserted into External Tools definitions.
$(Platform) actually already works, but is undocumented and not exposed in the UI pop-up menus. $(Configuration) does not. Please add full support for both of these, as these are incredibly useful, for example to enable easy custom runs of MSBuild against the current project to execute custom build targets.