Steps:
1) Search for airport pair.
2) Then click on the Partner Airlines toggle.
3) If partner airlines are available, they'd be shown either point-to-point or via a *A hub. Note if the airport code doesn't appear / isn't recognized, it doesn't mean the routing isn't available - you'd just have to call. eg. many Turkish cities are flown by TK, yet SQ's partner airline search doesn't produce any results. Also there's weird behavior like IST-MLA being shown, but MLA-IST isn't.
4) Once you've set the Partner Airlines in your initial search, subsequent searches within the same session would also show partner airlines first.