It's all in the e-mail they sent - here's the relevant part:
★"KK App" fan community app★
1 month plan 1,250 yen (tax included)
6 month plan 7,250 yen (tax included)
1 Year Plan 13,750 yen (tax included)
■"KK App" Monthly Paid Membership Benefits
・Participation in all KK App exclusive live broadcasts
・Comment on and view members-only chats
・Purchase app-only merchandise
・Ticket Advance Reservations and more…！！
▼For more information on the "KK App", click here!
https://www.koda-kumi-app.com
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EDIT:
The app itself isn't available outside of Japan from what I can tell (at least not for Android). So computer only for now.