The quickest way to add an attribute to an element is to proceed as follows:
Thanks to the auto-completion feature of this field, typing the first few characters of an attribute name is sufficient to specify it.
In many cases (ID, IDREF, enumeration, etc), this fields supports auto-completion too. However, unlike the Name field, the auto-completion feature here allows you to type anything you want, so you may have to press Space (which means: auto-complete as much as possible) in order to fully specify the value of the attribute.