Well, the reward is mostly the honor that your work is in the game. Even raletively popular MMOs aren't any more generous than this.
But Black Desert Online just showed us recently how it should be done, let's hope others catch on with this trend.
Ok Natsu gave you a good option but for whenever you want to play a Japanese/chinese/Korean game i recommend to use Qooapp from your mobile.
Then just look out on the Menu the Button that says Get QooApp and install.(you need to go to settings on your phone and accept to install apps from unknown sources)
After having installed Qoo open your app and search for the word Valkyrie and select the game and then Press Download, after you install an app via Qoo, the app will ensure you have the most recent versions of the apps you have in your phone specially those that are from Asian app-stores.
https://apkpure.com/google-chrome-fast-secure/com.android.chrome#download
Pick one of the older versions and see if your device lets you install as is. It may not, in which case you'd have to uninstall/disable updates on chrome and try again.
I actually just downloaded the apk from apkpure xD
Amazon link .. well, amazon JPN.
Fate/Grand Order. ~~From what I can tell it's only out in Japan so far.~~ Nevermind, appareantly it's only available in the West (meaning only in America for some odd reason) so everyone else outside are unable to play.