Just a simple email link is written as

mailto:email@email.com

So once you have this code for a link it will open the default email client with email@email.com in the TO field.

So now you want to show a default subject line. For that you can simply append ?subject=Your Subject to the above link. So it becomes

mailto:email@email.com?subject=YourSubject

Next thing could that you want to add some text in the body of email. For that you add body=Your Body to the above link. So now it becomes:

mailto:email@email.com?subject=YourSubject&body=YourBody

So here is a new thing I learnt today! I wanted to write the text in body but then I needed some text written on the next line. So I found an equivalent of
usually used in webpages. It is %0A . So now where you have the text for the body, if you add %0A, it moves the text to the next line. So now our last link would become:

mailto:email@email.com?subject=YourSubject&body=YourBody%0ANextLine

That is it! Hope this tip would be useful!