You need to enter "Luzern Bahnhofquai" to "Stansstad (See)".

There's no need to do so because you do just get a reduction. But it's possible. 

Thanks.

I tried that but the app says it can’t find that station.

You need to enter it exact that way with free spaces as indicated. If that doesn't work use just "Luzern" as departure station and set the filter for ferries only. That does work, too. But you need to enter "Stansstad (See)" (exactly like this) as arrival station, not just "Stansstad".

(see first possible date if that's the problem)

(unfortunately in german but it should look similar in english)

To travel on it though, you don't need to add it to your pass because you only get a discount.

Thanks. OK I got it to work and entered it as a journey. 

I already paid for the tickets (half discount) and I’m assuming that on boarding they will want to see my tickets and proof that I have a Eurail Pass. So do I just show the “My Pass” page or is there something else required ?

The My Pass page should be enough since that will show the validity period.