You don’t have it already?!
It’s pretty easy. If you’re on mobile just click this link here: https://mstry.co/app.
You can also search either app store for “Mystery Seattle” to find the app through there.
In case you just want the links, here they are:
Android (Google Play Store): https://play.google.com/store/apps/details?id=com.mysteries.mystery
IOS (Apple App Store): https://apps.apple.com/us/app/mystery/id1468331149