What will the Approve or Deny button do? Does it mark a checkbox, or a dropdown or something? Either way, you can make a formula Rich Text button and simply put that field in your notification. A couple of notes:
1. If the form is not set to EOTI, then whomever is approving or denying will need to be a QB and logged in. If not logged in, they first will go to login screen, then I believe the button should complete its task.
2. I know in Outlook the buttons can take some creative work to make look pretty. I found one way that I can share if you use Outlook, but I am sure others that understand CSS/HTML better than me would have even better suggestions.
3. As far as I know, when that button is in your email, that link forever stays live. Meaning, if days later they forget if they clicked the button, they can go back to that email and click it again, or click the other option. In my case, the button click I wanted was going to log their answer in a logged field. I was worried they would click the button multiple times and create needless duplicated logged answers. My solution was to have the rich text button actually open up the record, but to a different form. On that form I could use more buttons and form rules. Like - Click approve or deny (these are the real buttons). Then maybe a form rule that said, if approve or deny is selected, hide buttons and show text that says ' You have already completed this request'. Or something of the sort.
Someone smarter may have a better solution for #3.
