Custom file name to save the pdf using a button

[0experience]

Hello,

We can save a file where user need to give a file name

app.execMenuItem("SaveAs")

Now, I want a button, if user click then it will suggest user a pdf name based on the value given in the form. 

I am using the below code (based on this post) [on the button for click action]

var mySaveAs = app.trustedFunction(
   function(oDoc, cFlName) {
      app.beginPriv();
      
      try {
         oDoc.saveAs(cFlName);
      } catch (e) {
         app.alert("Error During Save");
      }

      app.endPriv();
   }
);

// Append the suffix "review" to the file name
var suggestedFileName = xfa.resolveNode("Num").rawValue + "_review";

mySaveAs(event.target, suggestedFileName + ".pdf");

But getting an error  

NotAllowedError: Security settings prevent access to this property or method.
App.trustedFunction:2:XFA:form1[0]:Page_1[0]:SBOR_Button_Subform[0]:Button_Save_All_Data[0]:click

Here, I used the trusted function's begin, and end privatization. But still why I am getting error, and how can I solve the error?

Update: I think I need to put the trusted function in my Adobe's JavaScript folder. However, this doesn't seems to be the best practice for my purpose, as I can't control my user's machine (and can't put the JS file on the Folder Level). Are there any other options to achieve the "Rename" functionality?

Thank you