You can't. Google looks for fares that everybody can buy. To get the WE fare, you first must identify yourself as a WE card holder, which you can only do on the AF site.

There are some other idiocies attached to the WE card fares. For instance that you cannot book for other people that don't have a weekend card. So if you want to travel with your kid you can't (kid doesn't have weekend card), so you want to book for the kid in a stand-alone booking, but that forces you to book a UM fare!