To complete a web form:

Go to the webpage that has the form.

Select a box on the form and start typing.
Safari tries to find information for the field, checking your address book and information you entered on previous forms. When Safari finds a match, it finishes entering the text for you. If it doesn’t find a match, you have to complete the field yourself.

If several items match what you typed, a menu appears. Select the correct item and press Return.