The text is wonkier than the word

For years I've wondered how to get students to reliably submit something I think of as standard text.  Not html or mime documents that will display differently according to what I view them with, but the modest expression of concepts using the 128 characters (mostly upper- and lower-case Latin alphabet, puncuation, digits, and special symbols) of the ascii format.

This becomes particularly maddening when all I want is a few dozen characters --- say the URL of a website.  How can I make sure that the URL isn't hidden deep inside the html formatting of somebody's favourite email app?

Something that works 95% of the time is to ask for the URL to be in the Subject: line of the email.  This line is forced to be in plain text by the mail software.  Sure, a few students will still ignore the instruction about the Subject: line, but most submissions will be in plain text.

My next challenge: how to communicate that the useful URL for a blogging site is the public address, not the address that the blog author uses to modify the site.