Thanks for your comments. They are very useful. Here are my responses to your question and comments.
"Do you need a form to set the values for the email?" Yes, I need a form for a user message, but I also want the email address that is contained in the Detail record, so that my user can send a message to him/her.
"You could also add the form to a new page and just have a recordset like the one on your details page that is filtered by the record id to get at the values." The question for me is how to do this. I need the email address from this one detail record, which as been selected by the user.
I have attached my attempt in the zip file, where I have created a new page, inserted Form Builder, Universal Email, created a new record set (which contains the email address), and selected UserID as the dynamic recipient, and then copied this page into my Detail page. Unfortunately, this does not go far enough. I do not know how to pass the user ID selected in the the Detail record to the behaviors in the new UA page.
I am able to successfully send an email with this new form from the Detail page, but it is sent to email address of the first record in my record set, apparently because I have not passed the Detail UserID in a query.
Thanks for your help.