Currently, you’re storing the input values in $0 and $1, but the line i(r0, r1).r2 may not properly assign the return value to $2

Change this line:

System::Call "$INSTDIR\NSISRegistryTool::Add(i, i) i(r0, r1).r2"

By:

System::Call "$INSTDIR\NSISRegistryTool::Add(i $0, i $1) i .r2"

https://nsis-dev.github.io/NSIS-Forums/html/t-309400.html