I have to send a form to some people and I do it with a google script. I try to embed the form in the email with this code : https://stackoverflow.com/a/23671529/4305236:
When Google sends the form to folks directly and embeds the form in the email they are using their new offering called AMP. You can read more about it here:
The code you're using gets the raw HTML of the form as it would be rendered for a user in a browser. This code is not in AMP format. Ergo it will not do what you want.
If you want that AMP experience you will need to create the AMP email yourself.
I don't think FormApp has a programatic way to send the form to folks, like you can from https://docs.google.com/forms/.
I hope that helps.