Within the New Message Content you already have the ability to insert the following:
{{subject}} = Subject
{{message}} = Full Message
{{message_url}} = URL of message
{{sender}} = Sender Name
{{receiver}} = Receiver Name
{{site_title}} = Website title
{{site_url}} = Website URL
I would like to insert in the same manner a special user profile field I created with “Ultimate Member” plugin called “One click Login”. I can see this field in the member profile area where I also see “First Name”, “Last Name”, “Nickname”, “Email”, “Website”, etc. My goal is to send a link that will automatically login the user so they can see their inbox. I am not giving users access to their account or password, they can only see their messages by using the One Click login link I have for them.