Use username in email template

I’m trying to send a custom email which contains the username in the HTML body. I’m using this:

 <p><strong>Your username: </strong>{{ user.username }}</p>

However, this comes up empty in the email, while my raw JSON for the specific users does have a field for username:

What am I doing wrong?

I guess that’s because of the fact that this property cannot be accessed at least that’s what this doc says:

The username property isn’t mentioned in the available properties list