Shift-clicking an item in chat (any item link), auction house or bags when at the auction house puts the name of that item to the search field and searches for it. Shift-alt-clicking simply appends the item name to the current search text.

Plain and Simple. No requirements, no options.

Note: If you shift click something while the chat edit box is open it will still be linked in chat. AH_QuickSearch only works when the chat edit box is closed.

Coincidentally this is very similar to Auction Lookup except this addon also allows shift-clicking links and items you just found in the AH browser.