Create an action after posting a vendor invoice journal from a purchase order though X++

Hello,

I'm working with Dynamics 365 Finance and Operations,

The requirement is that when someone posts a pending vendor invoice from a purchase order by clicking "Post" on the vendor invoice form, an Email has to be sent with the preview report of the invoice journal.

I already have the class created and working, the only thing left is make it run after the vendor invoice is posted.

I tried to make a class extension of the PurchFormLetter.run() method, the method is called when I post the PO, I need to get either the PurchTable purchId or the VendInvoiceJour record to sent them to my class, but I can't find the way to get them from PurchFormLetter, this is my code:

[ExtensionOf(classStr(PurchFormLetter))]
final class ATXPurchFormLetter_Extension
{
    public void run()
    {
        PurchTable                  purchTableAlt = this.purchTable();
        VendInvoiceJour             vendInvoiceJour;
        SendInvoiceReportEmail      emailEngine = new SendInvoiceReportEmail();

        info("before run");
        next run();
        info("after run");

        while select vendInvoiceJour where vendInvoiceJour.PurchId == purchTableAlt.PurchId
        {
            emailEngine.sendEmail(vendInvoiceJour);
        }
        
    }
}

I tried getting the purchTable() method to get PurchTable, but it's not returning any data.

Is there a way I can get those records from the class? Do I have to extend another class or method?