var setting: vscode.Uri = vscode.Uri.parse("untitled:" + "C:summary.txt");
vscode.workspace.openTextDocument(setting).then((a: vscode.TextDocument) => {
    vscode.window.showTextDocument(a, 1, false);
}, (error: any) => {
    console.error(error);
    debugger;
});

...
var setting: vscode.Uri = vscode.Uri.parse("untitled:" + "C:summary.txt");
vscode.workspace.openTextDocument(setting).then((a: vscode.TextDocument) => {
    vscode.window.showTextDocument(a, 1, false).then(e => {
        e.edit(edit => {
            edit.insert(new vscode.Position(0, 0), "Your advertisement here");
        });
    });
}, (error: any) => {
    console.error(error);
    debugger;
});
...