Hello. There is a article about how to Take and update message from Email Templates add make a custom click link for that email
First of all, You need to take Email from Message Email Templates by this way
var notification = (Notification)PXSelect<Notification, Where<Notification.notificationID, Equal<Required<Notification.notificationID>>>> .Select(Base, 14);

Then to select text from this Notification object by doing this. We are converting it to text from HTML text to operate with it.
var emailText = Tools.ConvertHtmlToSimpleText(notification.Body).TrimStart();

Before adding a custom link don’t forget to convert text to HTML. And then just replace text with HTML code.

To attach file for email, we could do next thing
I created method to convert any file to a byte array

And then in AddAttachment just using name of file that I need and converting it to byte array and attaching it to email.

After that you will have a custom link and also attached file in email body