I just went through this. I had to call United. I spoke with a customer service agent who was helpful but who seemed at a loss at multiple points during the process as to how to do it.

I was told that if award ticket availability appears on United's screens, they take the customer information and the then submit the request to Aloha, who then either confirms or denies it with 24 hours. Seemed to be quite a clunky way of doing it.

I couldn't get the seats I needed, but since I was trying to get 5k interisland award tickets (quite a bargain), it was worth a shot. Although it probably took 20 minutes or so on the phone to find this out.