Hello jsfernando,
In the construction section of the email field on lthe left side click on "Editor" next to "Message".
In the editor you can construct your message like this:
Hello #user_name# etc.
Where #ser_name is the field name of your username field. To let SEBLOD know that you want to use a field value, you have to embed the enclose field_name with the # character.
Hope this helps.