mailto: example with subject line
Here's how the mailto: URL should be composed:

    mailto:example@example.com?subject=Hello%20World

Replace example@example.com with your intended email address and replace the subject text with your desired subject, but remember to put in %20 in place of any spaces.  Some browsers may be able to automatically replace the spaces with %20 while some may not.
Thank you!
You're welcome! If you have any more questions, feel free to ask.