When adding a link to any other web page, you usually add
https:// followed by the rest of the url. For example,
Similarly when adding a link to an email address, you need to replace the http part with mailto: prefix followed by the email address you want to link to. Here is a example:
Here is how you will add it in a WordPress post.
Select the text you want to add the link to and press the insert link button (looks like a chain)
This will bring up the insert link popup where you need to add your mailto link. Click on the apply button to close the insert link popup.
WordPress will automatically detect that the text you are trying to link is an email address and will add the mailto: prefix to the email address.
You can add an email link to any text. If the text is not a valid email address then you will need to add your complete link. Like this: