To create a mailto link use the following HTML code:
<a href="mailto:hello@shihabiiuc.com">Email Me</a>
<!-- or -->
<a href="mailto:hello@shihabiiuc.com">hello@shihabiiuc.com</a>
These two lines create links labeled “Email Me” & “hello@shihabiiuc.com” respectively. If someone clicks any of the links, it will open the user’s default email client with the “to” field pre-populated with “hello@shihabiiuc.com.”
Create mailto link that prepopulates the email subject
See the following code to create an HTML mailto link that prepopulates the email subject.
<a href="mailto:hello@shihabiiuc.com?subject=About%20your%20service">Email Us</a>
After clicking the “Email Us” link, it will prepopulate the “to” & “Subject” fields. (as you see below)

Create mailto link that prepopulates the email body
<a href="mailto:hello@shihabiiuc.com?subject=About%20your%20service&body=Hello,%20How%20are%20you">Email Us</a>
It looks like the following screenshot after I clicked the email link:

Mailto link that prepopulates to, cc, bcc, subject & body (all email fields)
The following code prepopulates the to, cc, bcc, subject & body:
<a href="mailto:hello@shihabiiuc.com?cc=ccaddress@email.com&bcc=bccaddress@email.com&subject=About%20your%20service&body=Hello,%20How%20are%20you">Email Me</a>
Upon clicking the “Email Me” link, it looks like the screenshot below.

Conclusion
I showed you how you can prepopulate all the email fields one by one using the mailto link. You may need to customize the “Subject & Body” fields. If you have not noticed, here the “%20” creates the space.
If you have any questions related to the HTML mailto link, let me know.