How to call Digital Signature using JavaScript?

My requirement is to show warning message before signing the document without customizing the PDF.

For this I am creating custom menu to execute below action in sequence:

1. Execute JavaScript to show alert 
2. And then pop up digital signature

Below is the Javascript:

var DoMyMenuItem = app.trustedFunction(function(cItemName) {
    app.beginPriv();
    app.execMenuItem(cItemName);
    app.endPriv();
});

function CallMenu() {
    try {
        app.execMenuItem("FillSignApp");
        //DoMyMenuItem("Signatures");
    } catch (e) {
        app.alert("Error: " + e);
    }
}
app.addMenuItem({ cName: "My Action Menu", cParent: "Help", cExec: "CallMenu(); " });