how to program auto complete in Pages

There is no Pages specific feature that will do this. You have to set up a keyboard text replacement string, and then ensure that you have Edit menu ▸ Substitutions ▸ Text Replacment checked.

1. System Preferences : Keyboard panel : Text tab
2. Click the + symbol to add a new text string
   1. Replace column
      1. aus (double-click to enter)
      2. tab over to the with column
   2. With column

      Austin, TX 78730 (press return)

3. Quit System Preferences

In Pages v5.6 (current) type aus and when the substitution hint pops up, just press space, and continue on. Change your mind, click the x.

In Pages '09, the text substitution rules are in the application's Preferences Auto-Correction panel. There will be no Edit menu Substitutions Text Replacement menu item. You type the replacement text and follow it with the space bar, and the with text is instantly inserted. No prompt as in Pages v5.6.