How can I find out the value of $(ProjectDir)?

Open the project properties (Alter+Enter) and then go to "Build Events", select any of the build events, then select "Command Line", click "Edit", then press the "Macros >>" button.

Same here in Visual Studio 2019, you can find ProjectDir by clicking on Macros in additional Include Directories and then clicking on Macros (for me there was nothing in Build Events), like the answer in above Visual Studio 2015.