Variable substitutions that are available for tasks.json (see Integrate with External Tools via Tasks) do not include currently selected text. I am looking for something similar to $(CurText) that is available in Visual Studio Arguments for External Tools (see Managing External Tools).

{
    "version": "0.1.0",
    "tasks": [
        {
            "taskName": "ExternalProgram",
            "command": "externalprogram.exe",
            "isShellCommand": true,
            "args": [
                "${curtext}" // Or something similar
            ],
            "showOutput": "always"
        }
    ]
}

Please add a variable ${curtext} which would be replaced with the currently selected text in the editor window. If no selection is available, it could use the current line at cursor position.