I'm trying to pre-populate the subject line when an email address is clicked in our email message. Here's an example:
<a href="mailto:email@example.com?subject=This is a test">firstname.lastname@example.org</a>
When you click the email address, it only contains "This" in the subject line.
The href HTML tag won't let you have any spaces because it is treating it the same as it would a URL, which never contains spaces. Instead of spaces, you'll want to use the HTML translation of a space which is %20. So your example will look like this: