Fetch Email Id Associated with Username

I have a form that uses the "Users" look-up field, which is used to assign tasks to registered users on the app.

I would like to automatically send an email to the User when he/she s assigned a new task. "Username" is a text string, but I'd like to be able to fetch their "Email ID" so the email can be sent.

Doesn't seem like fetch records will work because the User info is not collected in another form.

Any solution to this? Looks like I can set the field type to "Email ID" but would prefer to use Username.

Thanks.