I have just checked my NOW TV box (haven't got a NOW Stick to hand) but this should be the same.
When you go into the Network Settings and select your Network wifi on the Stick and the enter box shows up on the screen with the on screen keyboard.
If you move to the far right on the keyboard and navigate down to the third line with a circle with !?# next to it and click OK on the remote it should change the on board keyboard screen to special characters including the * symbol.
Hopefully you have already figured this out.