You can do this by pasting the code listed below in an HTML editor:
<a href="mailto:email@example.com?Subject=Hello%20again" target="_top">Send Mail</a>
Replace firstname.lastname@example.org with your own e-mail address and replace the Send mail with your own text.
Where to add it
You will find the HTM editor in a text editor, for example, contents with a product (navigate to CATALOG > Products in the left menu and select a product), or on text pages, such as About us (reachable through SERVICE > Text pages). You can open the HTM editor by clicking HTML at the right at the top in the text editor menu. For more information, check the text editor support page.
If you prefer placing it in a specific location in your online store, you will have to use the Template editor. Only use this tool if you possess sufficient knowledge of HTML and CSS.