Hi Prashant Kumar,
Things have changed a lot from AX2012. You cannot easily create an approval Email as in AX2012 where users could directly respond to and approve or reject through an Email link. The only thing you can currently do is inserting a D365FO link into the Email that allows you opening the pending vendor invoices form.
I'm just working on a blog post that shows how to overcome those changes/shortcomings. Will keep you updated on that.
Best regards,
Ludwig