How can I set Xcode build setting's "Supported Platforms" to "iOS"

I tried the code below in my PostProcessBuild function. Looking at it in XCode, it seems correct at first, but when right clicking on the attribute, it shows that it sets it to an extra string value "iOS", but didn't actually choose iOS. So somehow I'm not setting the right value in the SetBuildProperty.

Any ideas? I'm surprised that no one has run into this yet....

PBXProjectpbxProject=newPBXProject();
pbxProject.ReadFromFile(projectPath);
string target =...

How can I set Xcode build setting's "Supported Platforms" to "iOS"