The template can be edited in the same place as other email template (Settings -> email yourforum.tld/admin/settings/email), you have to select the template contact-page.
It's not possible to embed it inside a post, sorry.
@Det-Prest Hello! Depends what plugin it is, we currently have support for paidmembershipspro and s3member, but ultimately any new plugins might need new plugins.
If you're on WordPress I'd personally just customize the sso-oauth plugin, and install the plugin on WordPress that allows it to act as an OAuth2 provider.